Control of the morphology of InGaN/GaN quantum wells grown by metalorganic chemical vapor deposition

Various techniques for morphological evolution of InGaN/GaN multiple quantum well (MQW) structures grown by metalorganic chemical vapor deposition have been evaluated. Atomic force microscopy, photoluminescence (PL) and X-ray diffraction measurements have been used for characterization. It is shown that inclusions, that are generated into the V-defects in the InGaN quantum wells (QW), can be removed by introducing a small amount of hydrogen during the growth of GaN barriers. This hydrogen treatment results in partial loss of indium from the QWs, but smooth surface morphology of the MQW structure and improved optical quality of InGaN wells are obtained. The density of the V-defects could be reduced by reducing the dislocation density of the underlying GaN buffer.     

Emission studies of InGaN layers and LEDs grown by plasma-assisted MBE

We prepared InGaN layers on GaN/sapphire substrates using rf-MBE. Photoluminescence (PL) from these layers, grown at different temperatures T_S, shows that there is a strong tendency of GaN to form a separate phase as T_S is increased from 600°C to 650°C. Concomitant with the phase separation, the PL from the InGaN phase broadens, which indicates that indium composition in this phase becomes increasingly non-uniform. Indium compositions measured by Rutherford backscattering (RBS) are consistent with these results. We also observed an increase in PL intensity for InGaN layers grown at higher temperatures. In this paper, we also report on preparing a top-contact InGaN/GaN light emitting diode. The device was operated at 447 nm and had the emission line width of 37 nm with no observable impurity related features. The turn-on voltage was 3.0 V. The output power was 20 μW at 60 mA drive current.     

Influence of Si doping on the optical and structural properties of InGaN films

Influence of Si doping on the optical and structural properties of InGaN epilayers with different Si concentrations was investigated in detail by means of high-resolution X-ray diffraction (HRXRD), scanning electron microscope (SEM), Cathodoluminescence (CL) and photoluminescence (PL). It was found that a small amount of Si doping in InGaN could enhance luminescence intensity, improve the crystal quality of InGaN and suppress the formation of V-defects in InGaN. Further investigation by CL showed that V-defects act as nonradiative center, which lower the luminescence efficiency of InGaN. Based on above-mentioned results, one possible mechanism of influence of Si doping on the formation of V-defects in InGaN was also proposed in this paper.     

Suppression of phase separation in InGaN layers grown on lattice-matched ZnO substrates

Sapphire and SiC are typical substrates used for GaN growth. However, they are non-native substrates and result in highly defective materials. The use of ZnO substrates can result in perfect lattice-matched conditions for 22% indium InGaN layers, which have been found to suppress phase separation compared to the same growths on sapphire. InGaN layers were grown on standard (0 0 0 2) GaN template/sapphire and (0 0 0 1) ZnO substrates by metalorganic chemical vapor deposition. These two substrates exhibited two distinct states of strain relaxation, which have direct effects on phase separation. InGaN with 32% indium exhibited phase separation when grown on sapphire. Sapphire samples were compared with corresponding growths on ZnO, which showed no evidence of phase separation with indium content as high as 43%. Additional studies in Si-doping of InGaN films also strongly induced phase separation in the films on sapphire compared with those on ZnO. High-resolution transmission electron microscopy results showed perfectly matched crystals at the GaN buffer/ZnO interface. This implied that InGaN with high indium content may stay completely strained on a thin GaN buffer. This method of lattice matching InGaN on ZnO offers a new approach to grow efficient emitters.     

Defect reduction in GaN epilayers and HFET structures grown on (0 0 0 1)sapphire by ammonia MBE

The quality of GaN epilayers grown by molecular beam epitaxy on substrates such as sapphire and silicon carbide has improved considerably over the past few years and in fact now produces AlGaN/GaN HEMT devices with characteristics among the best reported for any growth technique. However, only recently has the bulk defect density of MBE grown GaN achieved levels comparable to that obtained by MOVPE and with a comparable level of electrical performance. In this paper, we report the ammonia-MBE growth of GaN epilayers and HFET structures on (0 0 0 1)sapphire. The effect of growth temperature on the defect density of single GaN layers and the effect of an insulating carbon doped layer on the defect density of an overgrown channel layer in the HFET structures is reported. The quality of the epilayers has been studied using Hall effect and the defect density using TEM, SEM and wet etching. The growth of an insulating carbon-doped buffer layer followed by an undoped GaN channel layer results in a defect density in the channel layer of 2×10⁸ cm⁻². Mobilities close to 490 cm²/Vs at a carrier density of 8×10¹⁶ cm⁻³ for a 0.4 μm thick channel layer has been observed. Growth temperature is one of the most critical parameters for achieving this low defect density both in the bulk layers and the FET structures. Photo-chemical wet etching has been used to reveal the defect structure in these layers.     

Luminescence studies of defects and piezoelectric fields in InGaN/GaN single quantum wells

Transmission electron microscopy (TEM), cathodoluminescence in the scanning electron microscope (SEM-CL) and photoluminescence (PL) studies were performed on a 30 nm GaN/2 nm In_0.28Ga _0.72N/2 μm GaN/(0 0 0 1) sapphire single quantum well (SQW) sample. SEM-CL was performed at low temperatures ≈8 K, and at an optimum accelerating voltage, around 4–6 kV to maximise the quantum well (QW) luminescence. The CL in the vicinity of characteristic “V-shaped” pits was investigated. The near band edge (BE) luminescence maps from the GaN showed bright rings inside the boundaries of the pits while the QW luminescence maps showed pits to be regions of low intensity. These observations are consistent with TEM observations showing the absence of QW material in the pits. Variations in both the BE and QW maps in the regions between the pits are ascribed to threading edge dislocations. The CL and PL QW luminescence was observed to blue-shift and broaden with increasing excitation intensity. This was accompanied by decreasing spatial resolution in the CL QW maps implying an increasing carrier diffusion length in the InGaN layer. The reasons for this behaviour are discussed. It is argued that screening of the piezoelectric field in the material may account for these observations.     

Influence of various activation temperatures on the optical degradation of Mg doped InGaN/GaN MQW blue LEDs

GaN-based InGaN/GaN multi-quantum-well light emitting diode (MQW LED) structures were grown by metal organic chemical vapor deposition method. The optical properties of the LED structure have been investigated by using the photoluminescence and electroluminescence measurement. Both photoluminescence and electroluminescence results indicate that near pure InN clusters exist within the InGaN layers, which are responsible for the light emission in the LED. With increasing the Mg activation temperature of p-GaN layer, the optical properties of the LED structure tended to significantly degrade. This degradation was found to be deeply related to the variation of InN clusters in the active region. By the current–voltage measurement, a large forward voltage variation was observed. The voltage variation is caused to the conductivity variation of the p-GaN layer due to the different activation temperature. The turn-on voltage obtained from the best LED was 2.56 V and the forward voltage measured at 20 mA was 3.5 V. On the basis of these results, activation of the Mg-doped p-GaN layer must be carried out at the lowest possible value so as to obtain the better performance of LEDs.     

X-ray and cathodoluminescence study on the effect of intentional long time annealing of the InGaN/GaN multiple quantum wells grown by MOCVD

GaN-based InGaN/GaN multiple quantum wells (MQWs) structure having a high-quality epilayer and coherent periodicity was grown by metalorganic chemical vapor deposition. After thermal annealing of InGaN/GaN MQWs, the increase in temperature and annealing time caused the intermixing between the barrier and the wells, which in turn caused a decrease in periodicity on the high-resolution X-ray diffraction patterns. Thereby, we confirmed that the structural performance of InGaN MQWs is successively degrading with increasing thermal annealing temperature. Especially, InGaN MQWs of the sample annealed at 950 °C were profoundly damaged. The cathodoluminescence (CL) measurement indicated that MQWs emission intensity decreases with increasing thermal annealing temperature. Thus, the integrated CL intensity ratio of InGaN MQWs to GaN dramatically decreased while thermal annealing temperatures increased. This result caused the intermixing in MQWs to deteriorate the active layer performance. Furthermore, the peak position of MQWs showed a tendency of the red shift after high thermal annealing. It is suggested that the annealing-induced red shift in MQWs is attributed to the reduction of the inhomogeneity of the In content in the MQWs leading to the reduction of the quantized energies. Consequently, it indicates that the high temperature and the long-time thermal annealing would be inevitably followed by the structural destruction of InGaN MQWs.     

Influence of AlN nucleation layer growth conditions on quality of GaN layers deposited on (0 0 0 1) sapphire

The influence of AlN nucleation layer (NL) growth conditions on the quality of GaN layer deposited on (0 0 0 1) sapphire by organometallic chemical vapor phase epitaxy (OMVPE) has been investigated by X-ray diffraction, atomic force microscopy and transmission electron microscopy. Growth pressure, temperature and time were varied in this study. Results indicate that there exists an optimal thickness of the NL is required for optimal growth. Both thin and thick NLs are not conducive to the growth of high-quality GaN layers. Arguments have been developed to rationalize these observations.     

Profiling band structure in GaN devices by electron holography

Electron holography in a field emission gun transmission electron microscope has been used to profile the inner potential V₀ across GaN/x nm In_0.1Ga_0.9N/GaN/(0 0 0 1) sapphire samples (x=10, 40 nm) grown by molecular beam epitaxy and viewed in cross-section. Results are presented which suggest a decrease in V₀ of 3–4 V across the InGaN layer in the [0 0 0 1] direction. It is proposed that the results can be explained by charge accumulation across the InGaN layer and that the opposing contributions due to piezoelectric and polarisation fields are effectively masked by Fermi level pinning.     

Strain variation in p-GaN by different spacer layers in the light emitting diodes and their microstructural and emission behaviors

InGaN/GaN multiple quantum well-based blue light emitting diodes (LEDs) with different spacer layer structures were grown by metalorganic chemical vapor deposition. Fast-Fourier-transformed high-resolution transmission electron microscopy was used to determine the influence of the strain status in the spacer layer on Mg distribution and device performance. A comparison of the (1 1¯ 0 0) planar distance showed that the high-temperature grown InGaN layer in the spacer had a high level of stored strain. This led to the formation of a continuous facet contrast induced by Mg segregation in the p-layer, which was responsible for the deterioration of the electroluminescence performance of the LEDs. These results show that the delicate control of stored strain in nitride films is important for improving the device performance.     

Homoepitaxial growth and electrical characterization of GaN-based Schottky and light-emitting diodes

Homoepitaxial growth of GaN epilayers on free-standing hydride vapor phase epitaxy (HVPE) GaN substrates offered a better control over surface morphology, defect density, and doping concentration compared to conventional heteroepitaxial growth. The FWHM of the (0 0 0 2) X-ray diffraction (XRD) rocking curve from homoepitaxial GaN was measured to be as low as 79 arcsec, much smaller than 230 arcsec for GaN grown on sapphire. Schottky diodes grown on GaN substrates exhibited sharper breakdown characteristics and much lower reverse leakage than diodes on sapphire. However, the homoepitaxial devices had poor scalability due to the presence of yield-killing defects originating from the substrate surface. Vertical InGaN/GaN light-emitting diodes (LEDs) on GaN substrates showed reduced series resistance and reverse leakage compared to lateral LEDs on sapphire. Wafer mapping demonstrated that the distribution of leaky homoepitaxial devices correlated well with that of macroscopic defects in the GaN substrates.     

Electrical properties of photoanodically generated thin oxide films on n-GaN

The characteristics of photoelectrochemically (PEC) generated gallium oxide films on n-GaN using an 0.002 M KOH electrolyte are described. The chemical composition of the resistive layers was analyzed by Auger electron spectroscopy. The DC and HF characteristics of Al/Ti/PEC-Ga₂O₃ (gallium sesquioxide)/GaN structures were studied with current–voltage and capacitance–voltage measurements, respectively. Under reverse bias we found extremely low leakage currents (<10⁻⁸ Acm⁻² at −15 V) and a very low interface state density; high-temperature operation (up to 166°C tested) motivates the integration of the described dielectric layer forming technique into GaN based device process schemes. Our method may also be employed as gate recess technology.     

Role of vapor-phase diffusion in selective-area MOVPE of InGaN/GaN MQWs

Selective-area growth (SAG) of InGaN/GaN multiple quantum wells (MQWs) was performed by metalorganic vapor phase epitaxy (MOVPE). The layers of a blue light-emitting diode (LED), that includes five InGaN quantum wells, were grown on a patterned GaN template on a sapphire substrate. In order to elucidate the contribution of vapor-phase diffusion of group-III precursors to the in-plane modulation of luminescence wavelength, the width of a stripe selective growth area was 60 μm that is sufficiently larger than the typical surface diffusion length, with the mask width varied stepwise between 30 and 240 μm. The distribution of the luminescence wavelength from the MQWs was measured with cathode luminescence (CL) across the stripe growth area. The peak wavelength ranged between 420 and 500 nm. The peak shifted to longer wavelengths and became broader as the measured point approached to the mask edge. Such a shift in the peak wavelength exhibited parabolic profile in the growth area and the wider mask shifted the entire peak positions to longer wavelengths. These trends clearly indicate that the vapor-phase diffusion play a dominant role in the in-plane modulation of the luminescence wavelength in the SA-MOVPE of InGaN MQWs, when the size of a growth area and/or the mask width exceeds approximately 10 μm.     

Influence of AlGaN/GaN/InGaN superlattice on the characteristics of LEDs grown by metalorganic chemical vapor deposition

This study examined the influence of strain-compensated triple AlGaN/GaN/InGaN superlattice structures (SLs) in n-GaN on the structural, electrical and optical characteristics of LEDs by analyzing the etch pits density (EPD), stress measurement, high-resolution X-ray diffraction (HRXRD), sheet resistance, photoluminescence (PL) and light–current–voltage (L–I–V). EPD, stress measurement and HRXRD studies showed that the insertion of AlGaN/GaN/InGaN SLs during the growth of n-GaN effectively distributed and compensated for the strong compressive stress, and decreased the dislocation density in n-GaN. The operating voltage at 20 mA for the LEDs grown with SLs decreased to 3.18 V from 3.4 V for the LEDs grown without SLs. In addition, a decrease in the spectral blue shift compared to the LEDs grown without SLs was observed in the LEDs grown with the SLs.     

Lateral confined epitaxy of GaN layers on Si substrates

We developed a novel, simple procedure for achieving lateral confined epitaxy (LCE). This procedure enables the growth of uncracked GaN layers on a Si substrate, using a single, continuous metalorganic chemical vapor deposition (MOCVD) run. The epitaxial growth of GaN is confined to mesas, defined by etching into the Si substrate prior to the growth. The LCE-GaN layers exhibit improved morphological and optical properties compared to the plain GaN-on-Si layers grown in the same MOCVD system. By performing a set of LCE growth runs on mesas of varying lateral dimensions, we specified the crack-free range of GaN on Si as 14.0±0.3 μm.     

Experimental and theoretical investigations of the electrical properties of undoped and magnesium-doped GaN layers

The ac characteristics of GaN : Mg and undoped GaN layers, grown by MOVPE on sapphire substrates, are measured for a wide range of temperature and bias conditions, in order to investigate the effect of the magnesium-related level on the transport properties. Two peaks, whose height and position depend on the measurement temperature, are observed in the admittance curves (G/ω versus frequency) of the Mg-doped samples, whereas only one peak appears in undoped samples. The study of the frequency dependence of the impedance, with a model including the two metallic Au/GaN junctions, the GaN layer itself, shows that, besides the effect of the differential resistance of the layer which plays a role in both sample types, the presence of a Mg-related deep level contributes to the observed variations of the peaks in the admittance curves of the p-doped samples. Results of a theoretical steady-state and small-signal analysis based on numerical modelling of the Au/GaN/Au heterostructure complete our analysis.     

GaN/GaAs(1 0 0) superlattices grown by metalorganic vapor phase epitaxy using dimethylhydrazine precursor

Superlattices of cubic gallium nitride (GaN) and gallium arsenide (GaAs) were grown on GaAs(1 0 0) substrates using metalorganic vapor phase epitaxy (MOVPE) with dimethylhydrazine (DMHy) as nitrogen source. Structures grown at low temperatures with varying layer thicknesses were characterized using high resolution X-ray diffraction and atomic force microscopy. Several growth modes of GaAs on GaN were observed: step-edge, layer-by-layer 2D, and 3D island growth. A two-temperature growth process was found to yield good crystal quality and atomically flat surfaces. The results suggest that MOVPE-grown thin GaN layers may be applicable to novel GaAs heterostructure devices.     

Impedance spectroscopy analysis of AlGaN/GaN HFET structures

For HFET application a series of samples with 30 nm Al_xGa_1−xN (x=0.02–0.4) layers deposited at 1040°C onto optimised 2 μm thick undoped GaN buffers were fabricated. The Al_xGa_1−xN/GaN heterostructures were grown on c-plane sapphire in an atmospheric pressure, single wafer, vertical flow MOVPE system. Electrical properties of the Al_xGa_1−xN/GaN heterostructures and thick undoped GaN layers were evaluated by impedance spectroscopy method performed in the range of 80 Hz–10 MHz with an HP 4192A impedance meter using a mercury probe. The carrier concentration distribution through the layer thickness and the sheet carrier concentration were evaluated. A non-destructive, characterisation technique for verification of device heterostucture quality from the measured C–V and G–V versus frequency characteristics of the heterostructure is proposed.     

Spatially and spectrally resolved measurement of optical loss in InGaN laser structures

The optical loss co-efficient in InGaN laser diodes, emitting at 410 nm, has been measured. The measurement technique is based on the transmission of internally generated spontaneous emission through varying lengths of the laser waveguide. It is unique in that it provides spectral and spatial information on the optical loss. The lasers studied are typical of InGaN structures showing a high degree of waveguide loss, _i=40cm⁻¹. The measurements also show clear evidence of higher order transverse modes in the direction perpendicular to the growth plane with resonant leakage of the optical field into the outer layers of the structure. This produces a modulation in the loss of these modes.