Subband transitions in AlxGa1−xN /GaN/ AlxGa1−xN and AlxGa1−xN /InN/ AlxGa1−xN single quantum wells

We have calculated the spectral regime of subband transitions in Al_xGa_1−xN/GaN and Al_xGa_1−xN/InN single quantum wells. We used a simplified model to account for the internal electric fields, which modify the shape of the quantum well. Some of the parameters for these materials have not yet been firmly established. Therefore, we carried out the analysis for the extremes of the reported values of conduction band discontinuities and band gaps (in the case of InN). This analysis shows that the spectral regime of interband transitions for 1–4 nm thick wells has wavelengths above 0.5 μm for AlGaN/InN and above 0.8 μm for AlGaN/GaN and both heterostructures cover several μm wavelengths. The spectral variation with alloy composition is less pronounced in the Al_xGa_1−xN/InN single quantum wells due to the higher electric field present across the InN quantum well as compared to GaN. The results of these calculations are in good agreement with more rigorous theoretical approaches and available experimental values for Al_xGa_1−xN/GaN.     

High temperature electron transport properties of AlGaN/GaN heterostructures with different Al-contents

Electron transport properties in AlGaN/GaN heterostructures with different Al-contents have been investigated from room temperature up to 680 K. The temperature dependencies of electron mobility have been systematically measured for the samples. The electron mobility at 680 K were measured as 154 and 182 cm²/V·s for Al_0.15Ga_0.85N/GaN and Al_0.40Ga_0.60N/GaN heterostructures, respectively. It was found that the electron mobility of low Al-content Al_0.15Ga_0.85N/GaN heterostructure was less than that of high Al-content Al_0.40Ga_0.60N/GaN heterostructure at high temperature of 680 K, which is different from that at room temperature. Detailed analysis showed that electron occupations in the first subband were 75% and 82% at 700 K for Al_0.15Ga_0.85N/GaN and Al_0.40Ga_0.60N/GaN heterostructures, respectively, and the two dimensional gas (2DEG) ratios in the whole electron system were 30% and near 60%, respectively. That indicated the 2DEG was better confined in the well, and was still dominant in the whole electron system for higher Al-content AlGaN/GaN heterostructure at 700 K, while lower one was not. Thus it had a higher electron mobility. So a higher Al-content AlGaN/GaN heterostructure is more suitable for high-temperature applications.     

Interface traps contribution to capacitance of Al2O3/(GaN)AlGaN/GaN heterostructures at low frequencies

Influence of interface traps at Al₂O₃/(GaN)/AlGaN interface on low and high frequency capacitance of Al₂O₃/(GaN)/AlGaN/GaN heterostructure capacitor was studied. New features were observed in the capacitance curves. Obtained experimental results were modeled and simulated and accordance with the experiment has been obtained. For lower frequencies a new capacitance peak in the depletion and increase of the capacitance in a plateau region were measured. The capacitance peak in the depletion region was successfully explained by a capacitance response of the interface traps with U-shape density distribution. On the other hand the increase of the capacitance plateau was modeled by the homogeneous interface trap distribution. We assume that the traps located near the band edges having the highest density are able to respond to the low frequency measuring.     

Carrier transport mechanism of strained AlGaN/GaN Schottky contacts

Using polarization field effect-based thermionic field emission (PFE-TFE) model based on current–voltage–temperature data, possible carrier transport mechanisms for Pt/Au and Cr/Pd Schottky contacts to Al_0.25Ga_0.75N/GaN layers were investigated. Thermionic emission (TE) model was also investigated to compare to the PFE-TFE. It was shown that Schottky barrier heights (SBHs) are significantly affected by a polarization field-induced carrier density of the AlGaN layer. In addition, relatively little temperature dependence on the leakage current density of both contacts was found, which is in good agreement with the PFE-TFE model. The results indicate that the TFE is responsible for the current flow across the metal/AlGaN–GaN interface at T ≥ 293 K.     

Polarization and p-type doping effects on photoresponse of separate absorption and multiplication AlGaN solar-blind avalanche photodiodes

The photoresponse characteristics of separate absorption and multiplication (SAM) AlGaN solar-blind avalanche photodiodes (APDs) were investigated in detail. The p-i-n-i-n avalanche photodiodes were examined using the newly designed model of avalanche photodiodes in AlGaN. The research results showed that the dark current density was about 3.51 × 10^?8 A/cm², the light current density was 5.86 × 10^?5 A/cm² under near-zero bias, and the avalanche breakdown occurred at about 135.0 V under reverse bias, which were all consistent with the experimental data. To investigate the effects influencing the photoresponse characteristics of the APDs, their photo responsivity spectra under different biases were simulated. The APD featured a window region over the wavelength range from 260 to 280 nm with a high rejection ratio on the short-wavelength side. Meanwhile, the dependence of APD responsivity on the polarization charge revealed that the negative polarization charges strongly affected the responsivity. Increased negative polarization charges at the Al_0.4Ga_0.6N/Al_0.6Ga_0.4N interface markedly lowered the responsivity, whereas charges of the same polarity at the GaN/Al_0.4Ga_0.6N interface enhanced the responsivity. Furthermore, the dependence of responsivity on p-type doping was analyzed by comparison with the effects of negative polarization charges on the conduction band of the APDs. Finally, the inversion layer models are used to interpret the effects of these on the APD responsivity. This research is useful for exploring polarization and p-type doping effects in SAM AlGaN structures and realization of high responsivity solar-blind APDs.     

Electrical Characteristics of InGaN/AlGaN and InGaN/GaN MQW Near UV-LEDs

Electrical characteristics of In0.05 Ga0.95N/Al0.07Ga0.9aN and In0.05 Ga0.95N/GaN multiple quantum well （MQW） ultraviolet light-emltting diodes （UV-LEDs） at 400hm wavelength are measured. It is found that for InGaN/AlGaN MQW LEDs, both ideality factor and parallel resistance are similar to those of InGaN/GaN MQW LEDs, while series resistance is two times larger. It is suggested that the Al0.07Ga0.93N barrier layer did not change crystal quality and electrical characteristic of p-n junction either, but brought larger series resistance. As a result, InGaN/AlGaN MQW LEDs suffer more serious thermal dissipation problem although they show higher light output efficiency.     

Investigation of AlInN HEMT structures with different AlGaN buffer layers grown on sapphire substrates by MOCVD

We investigate the structural and electrical properties of Al_xIn_1–xN/AlN/GaN heterostructures with AlGaN buffers grown by MOCVD, which can be used as an alternative to AlInN HEMT structures with GaN buffer. The effects of the GaN channel thickness and the addition of a content graded AlGaN layer to the structural and electrical characteristics were studied through variable temperature Hall effect measurements, high resolution XRD, and AFM measurements. Enhancement in electron mobility was observed in two of the suggested Al_xIn_1?xN/AlN/GaN/Al_0.04Ga_0.96N heterostructures when compared to the standard Al_xIn_1–xN/AlN/GaN heterostructure. This improvement was attributed to better electron confinement in the channel due to electric field arising from piezoelectric polarization charge at the Al_0.04Ga_0.96N/GaN heterointerface and by the conduction band discontinuity formed at the same interface. If the growth conditions and design parameters of the Al_xIn_1?xN HEMT structures with AlGaN buffers can be modified further, the electron spillover from the GaN channel can be significantly limited and even higher electron mobilities, which result in lower two-dimensional sheet resistances, would be possible.     

Luminescence emission from Al0.3Ga0.7N/GaN multi quantum disc core/shell nanowire: Numerical approach

In this work, a numerical approach to investigate the room temperature luminescence emission from core/shell nanowire is presented where GaN quantum discs (QDiscs), periodically distributed in Al_xGa_1−xN nanowire, is considered as core and Al_xGa_1−xN as shell. Thin disc shaped (Ring shaped) n-doped region has been placed at the GaN/ Al_xGa_1−xN (Al_xGa_1−xN /air) interface in Al_xGa_1−xN region in axial (radial) directions. To obtain energy levels and related wavefunctions, self-consistent procedure has been employed to solve Schrodinger-Poisson equations with considering the spontaneous and piezoelectric polarization. Then luminescence spectrum is studied in details to recognize the parameters influent in luminescence. The results show that the amount of doping, size of QDiscs and theirs numbers have remarkable effects on the band to band luminescence emission. Our numerical calculations gives some insights into the luminescence emission of core/shell nanowire and exhibits a useful tool to analyze findings in experiments.     

AlGaN-based high-performance metal–semiconductor–metal photodetectors

Design, structure growth, fabrication, and characterization of high performance AlGaN-based metal–semiconductor–metal (MSM) photodetectors (PD) are reported. By incorporating AlN nucleation and buffer layers, the leakage current density of GaN MSM PD was reduced to 1.96 × 10⁻¹⁰ A/cm² at a 50 V bias, which is four orders of magnitude lower compared to control devices. A 229 nm cut-off wavelength, a peak responsivity of 0.53 A/W at 222 nm, and seven orders of magnitude visible rejection was obtained from Al_0.75Ga_0.25N MSM PD. Two-color monolithic AlGaN MSM PD with excellent dark current characteristics were demonstrated, where both detectors reject the other detector spectral band with more than three orders of magnitude. High-speed measurements of Al_0.38Ga_0.62N MSM PD resulted in fast responses with greater than gigahertz bandwidths, where the fastest devices had a 3-dB bandwidth of 5.4 GHz.     

Two-dimensional electron gas densities in AlGaN/AlN/GaN heterostructures

The dependence of two-dimensional electron gas (2DEG) density and distribution in an Al_xGa_1-xN/AlN/GaN heterostructure on the thicknesses of the Al_xGa_1-xN barrier layer and the AlN interfacial layer are investigated theoretically. A competitive contribution of the AlGaN and AlN layers to the 2DEG density is revealed. For an AlN interfacial layer thinner than a critical value d_cAlN, the 2DEG density is dominated by the AlGaN barrier and the 2DEG density increases with the increase of the AlGaN barrier thickness, as in the case of a simple AlGaN/GaN heterostructure. While the AlN interfacial layer will take the dominant contribution to the 2DEG density as its thickness exceeds d_cAlN. In this case, the increase of AlGaN barrier layer thickness leads to the decrease of the 2DEG density. Detailed calculations show that the critical AlN thickness increases with the increase of Al content in the AlGaN barrier. PACS 85.30.De; 73.40.Kp; 02.60.Cb     

The effects of GaN capping layer thickness on two-dimensional electron mobility in GaN/AlGaN/GaN heterostructures

In this paper we present a study of the effect of GaN capping layer thickness on the two-dimensional (2D)-electron mobility and the two-dimensional electron gas (2DEG) sheet density which is formed near the AlGaN barrier/buffer GaN layer. This study is undertaken using a fully numerical calculation for GaN/Al_xGa_1−xN/GaN heterostructures with different Al mole fraction in the Al_xGa_1−xN barrier, and for various values of barrier layer thickness. The results of our analysis clearly indicate that increasing the GaN capping layer thickness leads to a decrease in the 2DEG density. Furthermore, it is found that the room-temperature 2D-electron mobility reaches a maximum value of approximately 1.8×10³ cm² /Vs⁻¹ for GaN capping layer thickness grater than 100 Å with an Al_0.32Ga_0.68N barrier layer of 200 Å thick. In contrast, for same structure, the 2DEG density decreases monotonically with GaN capping layer thickness, and eventually saturates at approximately 6×10¹² cm⁻² for capping layer thickness greater than 500 Å. A comparison between our calculated results with published experimental data is shown to be in good agreement for GaN capping layers up to 500 Å thickness.     

Strain modification of AlGaN layers using swift heavy ions

Epitaxial AlGaN/GaN layers grown by molecular beam epitaxy (MBE) on SiC substrates were irradiated with 150 MeV Ag ions at a fluence of 5×10¹² ions/cm². The samples used in this study are 50 nm Al_0.2Ga_0.8N/1 nm AlN/1 μ m GaN/0.1 μ m AlN grown on SI 4H-SiC. Rutherford backscattering spectrometry/channeling strain measurements were carried out on off-normal axis of irradiated and unirradiated samples. In an as-grown sample, AlGaN layer is partially relaxed with a small tensile strain. After irradiation, this strain increases by 0.22% in AlGaN layer. Incident ion energy dependence of dechanneling parameter shows E ^1/2 dependence, which corresponds to the dislocations. Defect densities were calculated from the E ^1/2 graph. As a result of irradiation, the defect density increased on both GaN and AlGaN layers. The effect of irradiation induced-damages are analyzed as a function of material properties. Observed results from different characterization techniques such as RBS/channeling, high-resolution XRD and AFM are compared and complemented with each other to deduce the information. Possible mechanisms responsible for the observations have been discussed in detail.     

On the characteristics of AlGaN films grown on (111) and (001) Si substrates

High Al-content Al_xGa_1−xN films were deposited on (001) and (111) Si substrates at 1000 °C using high temperature AlN buffer layers. Experimental results show that Al_xGa_1−xN films grown on (111) Si substrates exhibit better crystalline quality than that in the films deposited on (001) Si substrates. Cracks were found in the high Al-content Al_xGa_1−xN/(111) Si samples but they were not observed in the Al_xGa_1−xN films grown on (001) Si substrates having the same film thicknesses and Al compositions. Based upon the results of X-ray diffraction (XRD) and transmission electron microscopy (TEM), it appears that mono-crystalline Al_xGa_1−xN films were achieved on (111) Si substrates while columnar structure was observed in the Al_xGa_1−xN/(001) Si samples. According to the depth profiles of Al_xGa_1−xN/Si samples using secondary ion mass spectroscopic (SIMS) analyses, enhanced Al inter-diffusion in the Al_xGa_1−xN/(001) Si samples was identified. Room temperature (RT) photoluminescence (PL) measurements of the Al_xGa_1−xN (x≦0.10)/(111) Si samples exhibit strong near band edge luminescence. The PL emission linewidth was found to decrease with the decrement of Al-content.     

AlGaN/GaN HEMT based hydrogen sensor with platinum nanonetwork gate electrode

AlGaN/GaN high electron mobility transistor (HEMT) based hydrogen sensors incorporating platinum nanonetworks in the gate region were demonstrated. Pt nanonetworks with 2–3 nm diameter were synthesized by a simple and low-cost solution phase method, and applied to the gate electrode of transistor sensor. The HEMT with physically and electrically connected Pt nanonetwork gate showed good pinch-off and modulation of drain current characteristics. Compared to conventional Pt thin film AlGaN/GaN HEMT sensor, the Pt nanonetwork sensor has dramatically improved current response to hydrogen. Relative current change of Pt nanonetwork gated sensor in 500 ppm H₂ balanced with Air ambient was 3.3 × 10⁶% at V_GS of ?3.3 V, while 2.5 × 10²% at V_GS of ?2.9 V for Pt film. This results from large increase in channel conductance induced by huge catalytic surface area of nanostructured Pt networks.     

The influence of AlN interlayers on the microstructural and electrical properties of p-type AlGaN/GaN superlattices grown on GaN/sapphire templates

AlN with different thicknesses were grown as interlayers (ILs) between GaN and p-type Al_0.15Ga_0.85N/GaN superlattices (SLs) by metal organic vapor phase epitaxy (MOVPE). It was found that the edge-type threading dislocation density (TDD) increased gradually from the minimum of 2.5×10⁹?cm^?2 without AlN IL to the maximum of 1×10¹⁰?cm^?2 at an AlN thickness of 20 nm, while the screw-type TDD remained almost unchanged due to the interface-related TD suppression and regeneration mechanism. We obtained that the edge-type dislocations acted as acceptors in p-type Al _x Ga_1?x N/GaN SLs, through the comparison of the edge-type TDD and hole concentration with different thicknesses of AlN IL. The Mg activation energy was significantly decreased from 153 to 70?meV with a?10-nm AlN IL, which was attributed to the strain modulation between AlGaN barrier and GaN well. The large activation efficiency, together with the TDs, led to the enhanced hole concentration. The variation trend of Hall mobility was also observed, which originated from the scattering at TDs.     

Deformation of AlGaN/GaN superlattice layers according to x-ray diffraction data

Three-crystal x-ray diffractometry is used for structural studies of nitride AlGaN/GaN superlattices (SLs) grown by metal-organic chemical vapor deposition on sapphire with GaN and AlGaN buffer layers with widely varied SL period (from 50 to 3500 Å), Al content in Al_x Ga_1?x N layers (0.1≤x≤0.5), and buffer layer composition. Satellite peaks characteristic of SLs are well pronounced up to the third order in θ-2θ scans of symmetric Bragg reflections and θ scans of the symmetric Laue geometry. The corresponding curves are well modeled by kinematic formulas. The average SL parameters, as well as the thickness, composition, and strain of individual layers, are determined using a combination of symmetric Bragg and Laue reflections. It is shown that all the samples under study are partially relaxed structures in which the elastic stresses between the entire SL and the buffer layer, as well as between individual layers, are relaxed. The AlGaN layers are stretched and the GaN layers are compressed. The GaN layer compression is larger in magnitude than the AlGaN layer tension because of thermoelastic stresses.     

Investigating gate metal induced reduction of surface donor density in GaN/AlGaN/GaN heterostructure by electroreflectance spectroscopy

The internal field of GaN/AlGaN/GaN heterostructure on Si-substrate was investigated by varying the thickness of an undoped-GaN capping layer using electroreflectance spectroscopy. The four samples investigated are AlGaN/GaN heterostructure without a GaN cap layer (reference sample) and three other samples with GaN/AlGaN/GaN heterostructures in which the different thickness of GaN cap layer (2.7 nm, 7.5 nm, and 12.4 nm) has been considered. The sheet carrier density (n_s) of a two-dimensional electron gas has decreased significantly from 4.66 × 10¹² cm⁻² to 2.15 × 10¹² cm⁻² upon deposition of a GaN capping layer (12.4 nm) over the reference structure. Through the analysis of internal fields in each GaN capping and AlGaN barrier layers, it has been concluded that the undiminished surface donor states (n_s) of a reference structure and the reduced n_s caused by the Au gate metal are approximately 5.66 × 10¹² cm⁻² and 1.08 × 10¹² cm⁻², respectively.     

Al2O3绝缘层的AlGaN/GaN MOSHEMT器件研究

在研制AlGaN/GaN HEMT器件的基础上,采用ALD法制备了Al₂O₃ AlGaN/GaN MOSHEMT器件.通过X射线光电子能谱测试表明在AlGaN/GaN异质结材料上成功淀积了Al₂O₃薄膜.根据对HEMT和MOSHEMT器件肖特基电容、器件输出以及转移特性的测试进行分析发现：所制备的Al₂O₃薄膜与AlGaN外延层间界面态密度较小,因而MOSHEMT器件呈现出较 关键词： 2O₃')" href="#">Al₂O₃ ALD GaN MOSHEMT     

Alloy disorder scattering limited mobility of two-dimensional electron gas in the quaternary AlInGaN/GaN heterojunctions

Nitride heterojunction field effect transistors (HFETs) with quaternary AlInGaN barrier layers have achieved remarkable successes in recent years based on highly improved mobility of the two-dimensional electron gases (2DEGs) and greatly changed AlInGaN compositions. To investigate the influence of the AlInGaN composition on the 2DEG mobility, the quaternary alloy disorder (ADO) scattering to 2DEGs in AlInGaN/GaN heterojunctions is modeled using virtual crystal approximation. The calculated mobility as a function of AlInGaN alloy composition is shown to be a triangular-scarf-like curved surface for both cases of fixed thickness of AlInGaN layer and fixed 2DEG density. Though the two mobility surfaces are quite different in shape, both of them manifest the smooth transition of the strength of ADO scattering from quaternary AlInGaN to ternary AlGaN or AlInN. Some useful principles to estimate the mobility change with the Al(In,Ga)N composition in Al(In,Ga)N/GaN heterojunctions with a fixed 2DEG density are given. The comparison between some highest Hall mobility data reported for Al_xGa_1−xN/GaN heterojunctions (x=0.06~0.2) at very low temperature (0.3~13 K) and the calculated 2DEG mobility considering ADO scattering and interface roughness scattering verifies the influence of ADO scattering. Moreover, the room temperature Hall mobility data of Al(In,Ga)N/AlN/GaN heterojunctions with ADO scattering eliminated are summarized from literatures. The data show continuous dependence on Hall electron density but independence of the Al(In,Ga)N composition, which also supports our theoretical results. The feasibility of quaternary AlInGaN barrier layer in high conductivity nitride HFET structures is demonstrated.     

Effects of DC and AC stress on the VT shift of AlGaN/GaN MIS-HEMTs

AlGaN/GaN MIS-HEMTs with adjusted V_T were fabricated using a recess gate to investigate the effect on actual operation when the polarity of the gate voltage is opposite in the on- and off-state. The direction and time exponents of V_T shift depend on the polarity of the gate bias stress. Electrons detrapping from the Al₂O₃/AlGaN interface trap site to AlGaN under negative gate bias stress has to overcome the energy barrier, resulting in a higher temperature dependence. In addition, the unaffected g_m and SS show that the degradation occurred primarily at the Al₂O₃/AlGaN interface rather than channel or mobility degradation. For unipolar and bipolar AC stresses, the time exponent of the V_T shift during stress time has two values, and a relatively low value during relaxation after bipolar AC stress. These results may be due to the further degradation by V_min at the broader energy levels of the Al₂O₃/AlGaN interface.