Optical and electrical characteristics of InGaP/AlGaAs/GaAs composite emitter heterojunction bipolar/phototransistors (CEHBTs/CEHPTs)

We report on new InGaP/AlGaAs/GaAs composite emitter heterojunction bipolar and phototransistors (CEHBTs/CEHPTs) with a low turn-on voltage. The composite emitter comprised of the digital graded superlattice emitter and the InGaP sub-emitter is used to smooth out potential spike associated with the emitter–base heterojunction and to obtain a low emitter–base turn-on voltage. A fabricated CEHBT exhibits a small offset voltage of 55 mV and a low turn-on voltage of 0.83 V with a dc current gain as high as 150. In case of a CEHPT’s collector–emitter characteristics with base floating, optical gains increase with increasing input optical power. Furthermore, the collector current saturation voltage is small due to a low turn-on voltage. We obtain an optical gain larger than 6.83 with a collector current saturation voltage smaller than 0.5 V. On the other hand, performance results of a CEHPT with two- and three-terminal configuration were investigated and compared.     

Low-frequency noise properties of p-type GaAs/AlGaAs heterojunction detectors

We have measured and analyzed, at different temperatures and bias voltages, the dark noise spectra of GaAs/AlGaAs heterojunction infrared photodetectors, where a highly doped GaAs emitter is sandwiched between two AlGaAs barriers. The noise and gain mechanisms associated with the carrier transport are investigated, and it is shown that a lower noise spectral density is observed for a device with a flat barrier, and thicker emitter. Despite the lower noise power spectral density of flat barrier device, comparison of the dark and photocurrent noise gain between flat and graded barrier samples confirmed that the escape probability of carriers (or detectivity) is enhanced by grading the barrier. The grading suppresses recombination owing to the higher momentum of carriers in the barrier. Optimizing the emitter thickness of the graded barrier to enhance the absorption efficiency, and increase the escape probability and lower the dark current, enhances the specific detectivity of devices.     

Photoluminescence investigation of Be-doped Npn AlGaAs/GaAs heterojunction bipolar transistor structures

Highly complex Npn AlGaAs/GaAs single heterojunction bipolar transistor (HBT) layers with Be-doped base were investigated by photoluminescence (PL) spectroscopy. Room temperature PL shows only a broad peak of GaAs due to thermalization; 15 K PL shows five peaks. The peak at 1.481 eV is from a p-type GaAs base, that at 1.517 eV is from a low-doped GaAs layer and that at 1.55 eV is from a high-doped GaAs collector. The that at 1.849 eV is due to bound exciton recombination in an AlGaAs emitter, and that at 1.828 eV is due to the acceptor-related transition from the AlGaAs layer. The integrated intensity ratio of these two peaks can be used to investigate the Be outdiffusion behavior, thus optimizing the growth conditions of base. The DC current gain of the HBT structure with different growth conditions was found to be in good agreement with the PL results.     

Study of persistent photoconductivity effect in n-type selectively doped AlGaAs/GaAs heterojunction

A detailed study of the persistent photoconductivity effect (PPE) at selectively doped Al_0.3Ga_0.7As/GaAs interface was carried out at low (4.2 and 77 K) temperatures on samples with different original channel concentrations and mobilities. The observed selectiveness of the PPE to the photon energy allowed us to identify two independent mechanisms of the PPE making almost equal contributions to the total effect. These two mechanisms are: (i) electron photoexcitation from DX centers in AlGaAs layer, (ii) electron—hole generation in bulk GaAs with a charge separation at the interface. It has been found that the behavior of the mobility as a function of the channel concentration (altered by light) depends on a setback thickness d. For a sample with small d a marked mobility drop has been found. The well-resolved structure in the dependence of the electron mobility on the channel concentration has been observed. The first peculiarity is explained by free electron population of AlGaAs layer due to the electron photorelease from DX-centers. The second feature, occurring at higher charge densities in the channel is attributed to the effect of intersubband scattering arising due to the electron occupation of an excited subband at the interface.     

Extrinsic base surface-passivated dual-emitter heterojunction phototransistors

N–p–n InGaP/GaAs Dual-Emitter HPTs (DEHPTs) with and without extrinsic base surface passivation were fabricated to investigate the influence of the surface leakage on the device’s optical performance. There are four operating regions appearing in the output characteristics of DEHPTs under illumination: negative-saturation, negative-tuning, positive-tuning and positive-saturation regions. The InGaP-passivated DEHPT (P-DEHPT), i.e. DEHPT with the extrinsic base surface passivated by InGaP, exhibits the maximum optical gains of 46.57, 46.86 and 47.39 while the non-passivated one (NP-DEHPT) shows ones of 32.02, 33.55 and 33.57 for optical powers of 8.62, 13.2 and 17.5 μW, respectively. However, the NP-DEHPT exhibits the larger peak gain-tuning efficiencies of 37.35, 41.03 and 44.10 compared to 12.76, 13.72 and 16.01 V ⁻¹ for the P-DEHPT for optical powers of 8.62, 13.2 and 17.5 μW, respectively. The better tuning efficiency makes the NP-DEHPT a possible low optical power optoelectronic application.     

Self-consistent calculation of Landau levels of a quasi-two-dimensional hole gas at a GaAs/AlGaAs p-type heterojunction

An efficient method is proposed for the self-consistent calculation of Landau levels of a quasi-two-dimensional hole gas at a GaAs/AlGaAs heterostructure in a perpendicular magnetic field. The method is based on transforming the Schroedinger and Poisson equations to a system of nonlinear differential equations which are then spatially discretized and solved by the method of relaxation. The method proposed is used to model the optical spectra for recombination of the quasi-two-dimensional hole gas with electrons localized at a dlayer of donors in an isolated p-type heterojunction. Particular attention is paid to effects associated with the dependence of the wave functions and shape of the potential well on the magnetic field, which have not been considered before. Fiz. Tverd. Tela (St. Petersburg) 40, 1117–1125 (June 1998)     

GaAs/AlGaAs超晶格的光致发光

在室温下测量了GaAs/A l0.3Ga0.7As超晶格的光致发光,发现在波长λ=761 nm处存在一较强的发光光峰,此发光峰目前尚未见报道。经理论分析表明,此峰是量子阱中的第一激发态电子与受主空穴复合发光。实验还观测到在λ=786 nm处,λ=798 nm处和λ=824 nm处分别存在一发光峰,分析表明λ=786 nm处的发光峰为量子阱阱中费米能级附近的电子与轻空穴复合发光;λ=798 nm处的发光峰为量子阱内的基态电子到轻空穴的复合发光;λ=824 nm处的发光峰为阱中激子复合复合发光。理论计算与实验结果符合的很好。     

Single quantum well transistor with modulation doped AlGaAs/GaAs/AlGaAs structures

Self-consistent calculations have been performed to obtain the wave functions and energy subbands of the two-dimensional electrons confined in a single quantum well of a Al_xGa_1?xAs/GaAs/Al_xGa_1?xAs heterostructure. The wave functions of the two-dimensional electron gas are found to be easily controlled by an external gate voltage applied between the AlGaAs-barriers, indicating a capability of fabricating a novel quantum well device, a modulation-doped single quantum well transistor.     

Hierarchical self-assembly of GaAs/AlGaAs quantum dots

A novel structure containing self-assembled, unstrained GaAs quantum dots is obtained by combining solid-source molecular beam epitaxy and atomic-layer precise in situ etching. Photo-luminescence (PL) spectroscopy reveals light emission with very narrow inhomogeneous broadening and clearly resolved excited states at high excitation intensity. The dot morphology is determined by scanning probe microscopy and, combined with single band and eight-band k.p theory calculations, is used to interpret PL and single-dot spectra with no adjustable structural parameter.     

Time-resolved photoluminescence of delta-doped AlGaAs/GaAs heterostructures

The photoluminescence (PL) at low temperature of three delta-doped AlGaAs/GaAs heterostructures is investigated under continuous and pulsed excitations. The PL under continuous excitations allows the identification of the trapping centres and probes the carrier's transfer to the GaAs channel. The time-resolved photoluminescence (TRPL) inquires about carrier dynamics and gives radiative lifetimes of different levels. The DX level shows two time constants of the intensity decay relating the splitting of the valence band under impurity strains which reduce the crystal symmetry. The two time constants evolve with temperature and exhibit an increase near T=50 K.     

Suppressed intermixing in InAlGaAs/ AlGaAs/GaAs and AlGaAs/GaAs quantum well heterostructures irradiated with a KrF excimer laser

The influence of gallium arsenide surface modification induced by irradiation with a KrF excimer laser on the magnitude of the quantum well (QW) intermixing effect has been investigated in InAlGaAs/AlGaAs/GaAs QW heterostructures. The irradiation in an air environment with laser pulses of fluences between 60 and 100 mJ/cm² has resulted in the formation of a gallium oxide-rich film at the surface. Following the annealing at 900 °C, up to 35 nm suppression of the band gap blue shift was observed in all the laser irradiated samples when compared to the non-irradiated samples. The origin of suppression has been discussed in terms of stress controlled diffusion. PACS 78.55.Et; 66.30.Lw; 73.21.Fg     

Investigation of optically generated kink effect in GaAs-based heterojunction phototransistors

Interaction between stacking faults (SFs) in pure Mg has been studied using density functional theory. The present results show that strong interaction between SFs can be extended up to several close-packed atomic layers. Especially, the interaction energies increase and decrease alternatively with the increase of SFs separation. Two distinct interaction models between SFs are proposed based on shear characteristics of SFs with different numbers of separating layers. The calculated electronic structures further reveal the underlying interaction mechanisms.     

Vertical and direct integration of heterojunction phototransistors and laser diodes

An optoelectronic integrated device based on the concept of vertical and direct integration is described. Four types of integrated devices composed of multiple heterojunction phototransistors and laser diodes are developed and the devices successfully achieve thirteen functions: optical switching, amplification, thresholding, bistability, light-controlled optical bistability, light-controlled optical thresholding, tristability, set and reset functions in tristable states, set and reset functions in tetrastable states, and bistable and tristable flip-flop functions.     

Managing disorder in random GaAs/AlGaAs superlattices

The artificial random Gaussian-type potential built in the GaAs/AlGaAs superlattices grown by molecular beam epitaxy was explored by various methods. The effect of the intentional disorder was shown to dominate intrinsic superlattice imperfections and its impact on the electronic properties was found to be in good agreement with the theoretical predictions. It was demonstrated that the modern state of the molecular beam epitaxy allows for a growth of the superstructured materials with well-defined disorder strength.     

Surface-emitting second-harmonic generation in AlGaAs/GaAs waveguides

We investigate the impact of waveguide properties on the performance of surface-emitting second-harmonic generation (SESHG) devices. Using multi-layer AlGaAs/GaAs ridge waveguides that are optimized for SESHG at fundamental wavelengths = 1060 and 1550 nm, we achieve nonlinear cross-sections among the highest reported so far. In these devices, characteristics of the guided fundamental wave strongly determine the SESHG performance: multiple reflections in the longitudinal direction affect the conversion efficiency while higher order lateral waveguide modes modify the SESHG farfield. Both effects are significant for applications of SESHG in integrated photonic circuits.     

p-type ohmic contacts to AlGaAs/GaAs heterostructures

Ohmic contacts to p-AlGaAs/GaAs heterostructures have larger resistivities compared to n-type structures because holes are heavier than electrons and the heterointerface barrier for high mobility structures is thicker. This work shows that diffusion of Zn during an alloy process of a Au/Zn/Au metallization establishes high acceptor concentrations throughout the heterostructure resulting in a thin metal-semiconductor barrier and probably a degradation of the heterointerface barrier by disordering, while the heterostructure outside the contact region remains unchanged. Contact resistivities down to 3.5ωmm at 25K could thus be achieved.     

Smith Purcell effect in GaAs/AlGaAs heterostructures

We report a new experimental technique to study the form of the hot electron distribution function in GaAs/AlGaAs heterostructures. A weak periodic surface potential induces Smith-Purcell-FIR-radiation of the electric field driven hot electrons in the 2-dimensional electron gas directly reflecting their velocity distribution. The FIR- radiation is detected by a magnetic field tuned InSb-detector. In samples with very low electron concentration and high mobility the emission spectra show a significant shift to higher energies and develop a steep high energy slope with increasing electric field when we use the geometry with grating vector q directed parallel to the electric field (q ∥ E). In the geometry q ⊥ E smooth decays are observed at lower energies. Comparison of the results with theory gives experimental evidence of a non-equilibrium shape of the distribution caused by the onset of LO-Phonon emission. In addition, the hot electron mean free path of the heated distribution is derived by investigating the experimental emission spectra as a function of the grating period length. The influence of a limited hot electron mean free path on the spectral width is described in terms of a Fourier-analysis of the interaction potential. In drift direction a mean free path of λ = 200 nm is obtained, whereas the mean free path is smaller in the direction perpendicular to the drift direction.     

Quantum cascade electroluminescence in GaAs/AlGaAs structures

The operation of a unipolar quantum cascade light-emitting diode based on the material system GaAs/AlGaAs is reported. The LED operates at a wavelength of 6.9 μm. Detailed analysis of the electroluminescence spectra shows a linewidth as narrow as 14 meV at cryogenic temperatures, increasing to 20 meV at room temperature. For typical drive-current densities of 1 kA/cm² the optical output power lies in the ten 10 nW range. Additional absorption and photocurrent measurements provide a complete characterization of the mid-infrared emitter.