High C-doping of MOVPE grown thin AlxGa1−xAs layers for AlGaAs/GaAs interband tunneling devices

High hole concentrations in LP-MOVPE grown GaAs and AlGaAs layers can be achieved by intrinsic C-doping using TMGa and TMAl as carbon sources. Free carrier concentrations exceeding 10²⁰ cm⁻³ were realised at low growth temperatures between 520–540°C and V/III ratios <1.2. The C-concentration increases significantly with the Al-content in Al_xGa_1−xAs layers. We observed an increase in the atom- and free carrier concentration from 5·10¹⁹ cm⁻³ in GaAs to 1.5·10²⁰ cm⁻³ in Al_0.2Ga_0.8As for the same growth conditions. Interband tunneling devices with n-type Si and p-type C-doped AlGaAs layers and barriers made of Al_0.25Ga_0.26In_0.49P have been investigated.     

OMVPE growth of In0.53Ga0.47As on InP using tertiarybutylarsine

We have successfully grown bulk In_0.53Ga_0.47As on InP using tertiarybutylarsine (TBA), trimethylindium and trimethylgallium. The growth temperature was 602° and the V/III ratio ranged from 19 to 38. Net carrier concentrations were 2 – 4 × 10¹⁵ cm^-3, n-type, with a peak 77 K mobility of 68,000 cm²/V. sec. Increasing compensation was observed in In_0.53Ga_0.47As grown at higher V/III ratios. PL spectra taken at 5 K revealed strong near bandgap emission at 0.81 eV—with the best sample having a FWHM of 2.5 meV. At lower energies, donor-acceptor pair transitions were evident. Strong and sharp 5 K PL emission was observed from InP/In_0.53Ga_0.47As/InP quantum wells grown with TBA.     

Effects of growth interruption on the optical properties of AIGaAs/GaAs and GaAs/InGaAs quantum wells grown by atmospheric pressure organometallic chemical vapor deposition

In this study, the effects of growth interruptions on Al_0.17Ga_0.83As/GaAs and GaAs/ In_xGa_1-xAs quantum wells (QWs) grown by organometallic chemical vapor deposition (OMCVD) were assessed using low-temperature photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies. Growth interruption times were varied between 60, 10, and 0 sec. For both material systems, as the interruption time was reduced, the ground-state QW transition energies increased, while the linewidths of the peaks decreased. For the Al_0.17Ga_0.83As/GaAs structures, 5 K PL data suggests that the incorporation of impurities is enhanced by longer growth interruption times. In addition, as the interruption time was reduced, the energy separation between the 5 K PL and PLE peaks (Stokes shift) decreased, and was as low as 2.6 meV for no interruption. For GaAs/In_0.11Ga_0.89As samples, 2 K PL data indicated that the incorporation of donor species was not a function of the growth interruption time.     

Optical and transport properties of δ-doped pseudomorphic AlGaAs/InGaAs/GaAs structures

We investigate the effects of spacer layer thickness on the optical and transport properties of the n-typeδ-doped pseudomorphic Al_0.30Ga_0.70As/In_0.15Ga_0.85As / GaAs structures. Aδ-doped AlGaAs/InGaAs/GaAs structure with a 6nm spacer layer yields a sheet carrier concentration of 1.5×10¹² cm^?2 at 77K with electron mobility of 6.4×10³ cm²/Vs, 3.11×10⁴ cm²/Vs, and 3.45×10⁴ cm²/Vs at room temperature, 77 and 20K, respectively. The effects of the different scattering mechanisms on luminescence linewidth and electron mobility have also been discussed.     

Hall analysis of the electrical properties of n-type AlxGa1-xAs grown by MOVPE

Variable temperature Hall measurements were used to study the electrical properties of undoped and Se-doped Al_xGa_1-xAs (0 <x < 0.4) layers grown by metalorganic vapour phase epitaxy (MOVPE). It is shown that the deep donor activation energy measured in undoped AlGaAs exhibits a similar dependency upon composition as that reported for Si-doped AlGaAs grown by MBE. For Al_xGa_1-xAs, doping with selenium is found to reduce the activation energy from 66 meV (forn = 4.1 x 10¹⁶/cm³), to 9 meV (forn = 1.6 × 10¹⁸/cm³).     

Effect of structural parameters on InGaAs/InAlAs 2DEG transport characteristics

The effect of structural parameters on the transport characteristics from 15 to 300 K of molecular beam epitaxy-grown InGaAs/InAlAs two dimensional electron gas structures lattice-matched to InP is determined. The InAlAs buffer layer thickness was varied from 1000 to 10,000Å. One sample also incorporated a InGaAs/InAlAs superlattice. The buffer layer thickness and structure had almost no effect on the mobility or sheet density. The InAlAs spacer layer was varied from 25 to 200Å. Increases in the InAlAs spacer thickness resulted in a monotonically decreasing sheet density and a peak in the mobility versus spacer thickness at 100Å. The highest 77 K mobility was 66,700 cm²/V/sds withN _D =1.2×10¹² cm^?2. The effect of illumination and temperature on the sheet concentration in these structures as well as on “bulk” InAlAs:Si was much smaller than in Al _x Ga_1?x As/GaAs structures or “bulk” Al _x Ga_1?x As, forx?0.30, indicating that devices based on this material system will not be characterized by many of the device instabilities observed in the AlGaAs/GaAs system.     

Electrical characterization of self-assembled In0.5Ga0.5As/GaAs quantum dots by deep level transient spectroscopy

We have investigated electron emission from self-assembled In_0.5Ga_0.5As/GaAs quantum dots (QDs) grown by molecular-beam epitaxy (MBE). Through detailed deep level transient spectroscopy comparisons between the QD sample and a reference sample, we determine that trap D, with an activation energy of 100 meV and an apparent capture cross section of 5.4×10⁻¹⁸ cm², is associated with an electron quantum level in the In_0.5Ga_0.5As/GaAs QDs. The other deep levels observed, M1, M3, M4, and M6, are common to GaAs grown by MBE.     

Gas-Source molecular beam epitaxy and characterization of inGaAs/lnGaAsP quantum well structures on InP

In this paper, we report the effect of using a group-V residual source evacuation (RSE) time on the interfaces of InGaAs/lnGaAsP quantum wells (QWs) grown by gas-source molecular beam epitaxy. High-resolution x-ray rocking curve and low-temperature photoluminescence (PL) were used to characterize the material quality. By optimizing the RSE time, a PL line width at 15K as narrow as 6.6 meV is observed from a 2 nm wide single QW, which is as good as or better than what has been reported for this material system. Very sharp and distinct satellite peaks as well as Pendellosung fringes are observed in the x-ray rocking curves of In_xGa_1−xAs/InxGa_1−xAS_yP_1−y multiple QWs, indicating good crystalline quality, lateral uniformity, and vertical periodicity. Theoretical considerations of the PL linewidths of In_xGa_1−xAs/InxGa_1−xAS_yP_1−y single QWs show that for QW structures grown with the optimized RSE time, the PL linewidth is mainly due to alloy scattering, whereas the contribution from interface roughness is small, indicating a good interface control.     

Effect of growth conditions on incorporation of Si into Ga and As sublattices of GaAs during molecular-beam epitaxy

The effect of dopant concentration and growth-surface crystallographic orientation on the incorporation of Si into Ga and As sublattices was investigated during GaAs molecular-beam epitaxy. The epitaxial layers (epilayers) were grown on GaAs substrates with (100), 2°(100), 4°(100), and 8°(100) orientations at a temperature of 520°C and with (111)A, 2°(111)A, 2°(111)A, 5°(111)A, 6°(111)A, and 8°(111)A (where A = Ga) orientations at a temperature of 480°C. The Sidopant concentration was varied within 10¹⁷–10¹⁹ cm^?3. Through electrical and photoluminescent methods of investigation, the Si impurity was found to occur at the sites of both GaAs-layer sublattices not only as simple donors and acceptors (Si_Ga and Si_As), but also as Si_Ga-Si_As, Si_Ga-V_Ga, and Si_As-V_As complexes. The concentration of Si impurity in various forms depends on the doping level of the layers and on the growth-surface orientation. Amphoteric properties of Si manifest themselves more prominently on the (111)A face than on the (100) one. It is shown that impurity defects form at the stage of layer crystallization and depend on the growth-surface structure.     

Persistent photo-conductance and photoquenching of selectively doped Al0.3Ga0.7As GaAs/heterojunctions

The dependence on photon energy of the persistent photoconductivity (PPC) in selectively doped high mobility Al_0.3Ga_0.7As—GaAs heterostructures has been measured at temperatures below 80 K. A decrease in conductivity due to light exposure at one wavelength after exposure to light at another wavelength — photo-quenching — is also found. It is concluded that deep centers in GaAs and AlGaAs other than the DX center in AlGaAs are mainly responsible for PPC.     

Characteristics of Si3N4/GaAs metal-lnsulator-semiconductor interfaces with coherent Si/Al0.3Ga0.7As interlayers

Si₃N₄/GaAs metal-insulator-semiconductor (MIS) interfaces with Si(10Å)/ Al_0.3Ga_0.7As (20Å) interface control layers have been characterized using capacitance-voltage (C-V) and conductance methods. The structure was in situ grown by a combination of molecular beam epitaxy and chemical vapor deposition. A density of interface states in the 1.1 × 10¹¹ eV^-1 cm^-2 range near the GaAs midgap as determined by the conductance loss has been attained with an ex situ solid phase annealing of 600°C in N₂ ambient. A dip quasi-static C-V demonstrating the inversion of the minority-carrier verifies the decent interface quality of GaAs MIS interface. The hysteresis and frequency dispersion of the MIS capacitors were lower than 100 mV, some of them as low as 50 mV under a field swing of about ±2 MV/cm. The increase of the conductance loss at higher frequencies was observed when employing the surface potential toward conduction band edge, suggesting the dominance of faster traps. Self-aligned gate depletion mode GaAs metal-insulator-semiconductor field-effect transistors with Si/Al_0.3Ga_0.7As interlayers having 3 μm gate lengths exhibited a transconductance of about 114 mS/mm. The present article reports the first application of pseudomorphic Si/ Al_0.3Ga_0.7As interlayers to ideal GaAs MIS devices and demonstrates a favorable interface stability.     

Growth of AlxGa1−x as by metalorganic chemical vapor deposition using trimethylgallium and trimethylamine alane

High-quality Al_xGa_1−xAs layers with aluminum arsenide contentx up to 0.34 have been grown in a low pressure metalorganic chemical vapor deposition (MOCVD) system using trimethylgallium (TMG), trimethylamine alane (TMAA) and arsine. The carbon content in these films depended on growth conditions but was in general lower than in those obtained with trimethylaluminum (TMA) instead of TMAA in the same reactor under similar conditions. Unlike TMA grown layers, the TMAA grown Al_xGa_1−xAs layers, (grown at much lower temperature—down to 650° C), exhibited room temperature photolu-minescence (PL). Low temperature (25 K) PL from these films showed sharp bound exciton peaks with a line width of 5.1 meV for Al_0.25Ga_0.75As. A 39 period Al_0.28Ga_0.72As (5.5 nm)/GaAs (8.0 nm) superlattice grown at 650° C showed a strong PL peak at 25 K with a line width of 5.5 meV attesting to the high quality of these layers.     

Si-defect concentrations in heavily Si-DOPED GaAs: annealing-Induced changes-II

Isothermal annealing produces changes in the free carrier density, defect-induced localized vibrational mode (LVM) infrared absorption, microstructure as measured by transmission electron microscopy (TEM), and critical resolve shear stress of heavily Si-doped GaAs. The changes have been measured and correlated for three different Si concentrations for several annealing temperatures. The measurements reveal temperature dependent annealing-induced changes in several specific defect concentrations. The observations indicate the following behavior for two ingots with [Si] ≳ 2 × 10¹⁹ cm⁻¹³: (1) when the anneal temperature, T_A = 400°C, the concentration of Si_ga donors, as determined from LVM spectra decreases probably due to the generation of V_Ga defects followed by the formation of Si_Ga-V_Ga pairs. This change is responsible for observed decreases in carrier density and the large increase in yield stress. The yield stress shows a dependence of the form σ-σ_o ∝ [Si_Ga-V_Ga]^1/4. (2) When T_A = 500°C, the LVM spectra indicate that all of the observed Si defect concentrations change. The decrease in [Si_Ga] alone cannot explain the decrease in carrier density, and a previous suggestion that a new acceptor is required is confirmed. Both the LVM measurements and the shear stress indicate that only a small fraction of the [Si_Ga] reduction is by the formation of Si_Ga-V_Ga pairs. (3) When T_A = 700°C, a new acceptor is still required and the other experimental observations at T_A = 500°C are also still seen here. There is a large decrease in [Si_Ga] and [Si_As] observed for short anneal times which coincides with the formation of Si-rich extrinsic loops and the loop area/vol increases with [Si]. (4) When T_A > 700°C, all of the changes become smaller as T_A increases. For lower [Si] ~ 1.5 × 10¹⁸ cm⁻³, no significant annealing-induced changes are observed for any of the T_A given above.     

Group-V composition control for InGaAsP grown by gas source molecular beam epitaxy

Based on our kinetics models for gas source molecular beam epitaxy of mixed group-V ternary materials, the group-V composition control in In_yGa_1−yAs_1−xP_x epilayers has been studied. The P or As composition in In_yGa_1−yAs_1−xP_x (lattice matched to InP or GaAs) can be obtained from a simple equation for substrate temperatures below 500°C. This has been verified by a series of experimental results.     

Reduced light sensitivity and persistent photoconductivity in novel modulation doped heterostructures incorporating an n·GaAs/AlGaAs superlatt1ce as the high bandgap material

A short period 15 Å/15 Å GaAs/Al_xGa_1?xAs superlattice where only the GaAs layers are doped with Si donors has been incorporated in a GaAs/Al_xGa_1?xAs molecular beam epitaxial modulation doped heterostructure in place of the doped high bandgap ternary alloy. The removal of the donors from the Al_xGa_1?xAs eliminates the component of light sensitivity resulting from the deep persistent photoconductivity (PPC) traps present in this material. This results in a reduction in overall light sensitivity and an elimination of the PPC effect at 77K.     

Photoreflectance spectroscopy of low-dimensional GaAs/AlGaAs structures

Results of room-temperature photoreflectance measurements on three GaAs/Al_0.33Ga_0.67As multiquantum well (MQW) structures with three different widths of wells and on two GaAs/Al_0.33Ga_0.67As high-electron-mobility transistor (HEMT) structures are presented. Energy-gap-related transitions in GaAs and AlGaAs were observed. The Al content in AlGaAs was determined. MQW transition energies were determined using the first derivative of a Gaussian profile of the measured resonances. In order to identify the transitions in the MQS, the experimentally observed energies were compared with results of the envelope function calculation method for a rectangular quantum well. The Franz–Keldysh oscillation (FKO) model was also used to determine the built-in electric field in various parts of the investigated structures. The values of the electric fields allow us to hypothesise about the origin of these fields. © 1997 John Wiley & Sons, Ltd.     

Hall Effect Studies of AlGaAs Grown by Liquid-Phase Epitaxy for Tandem Solar Cell Applications

We report results from Hall effect studies on Al _x Ga_1?x As (x = 0.23–0.24) with bandgap energies of 1.76 ± 0.01 eV grown by liquid-phase epitaxy (LPE). Room-temperature Hall measurements on unintentionally doped AlGaAs revealed p-type background doping for concentrations in the range 3.7–5.2 × 10¹⁶ cm^?3. Sn, Te, Ge, and Zn-doped AlGaAs were also characterized to study the relationship between doping concentrations and the atomic fractions of the dopants in the melt. Temperature-dependent Hall measurements were performed to determine the activation energies of the four dopants. Deep donor levels (DX centers) were dominant for Sn-doped Al_0.24Ga_0.76As, but not for Te-doped Al_0.24Ga_0.76As. Comparison of the temperature-dependent Hall effect results for unintentionally and intentionally doped Al_0.24Ga_0.76As indicated that the impurity contributing to the p-type background doping had the same activation energy as Mg. We thus suggest a Te-doped emitter and an undoped or Ge-doped base to maximize the efficiency of Al _x Ga_1?x As (x ～ 0.23) solar cells grown by LPE.     

Comparative study on the influence of Al component at GaAlAs layer for GaAs/AlGaAs photocathode

We designed two transmission-mode GaAs/AlGaAs photocathodes with different AlxGa_1-xAs layers, one has an Al_xGa_1-xAs layer with the Al component ranging from 0.9 to 0, and the other has a fixed Al component 0.7. Using the first-principle method, we calculated the electronic structure and absorption spectrum of Al_xGa_1-xAs at x=0, 0.25, 0.5, 0.75 and 1, calculation results suggest that with the increase of the Al component, the band gap of Al_xGa_1-xAs increases. Then we activated the two samples, and obtained the spectral response curves and quantum efficiency curves; it is found that sample 1 has a better shortwave response and higher quantum efficiency at short wavelengths. Combined with the band structure diagram of the transmission-mode GaAs/AlGaAs photocathode and the fitted performance parameters, we analyze the phenomenon. It is found that the transmission-mode GaAs/AlGaAs photocathode with variable Al component and various doping structure can form a two-stage built-in electric field, which improves the probability of shortwave response photoelectrons escaping to the vacuum. In conclusion, such a structure reduces the influence of back-interface recombination, improves the shortwave response of the transmission-mode photocathode.     

Large area depositon of Cd1-xZnxTe on GaAs and Si substrates by metalorganic chemical vapor deposition

Results of large-area (up to 1000 cm²/run) Cd_1-xZn_xTe heteroepitaxy on both GaAs and GaAs/Si substrates by metalorganic chemical vapor deposition (MOCVD) are presented. Cd_1-xZn_xTe (x = 0-0.1) films exhibited specular surface morphology, 1% thickness uniformity (standard deviation), and compositional uniformity (Δx) of ±0.002 over 100 mm diam substrates. For selected substrate orientations and deposition conditions, the only planar defects exhibited by (lll)B Cd_1-xZn_xTe/GaAs/Si films were lamella twins parallel to the CdTe/GaAs interface; these do not propagate through either the Cd_1-xZn_xTe layer or subsequently deposited liquid phase epitaxy (LPE) HgCdTe layer(s). Background Ga and As-impurity levels for Cd_1-xZn_xTe on GaAs/Si substrates were below the secondary ion mass spectroscopy detection limit. Preliminary results of HgCdTe liquid phase epitaxy using a Te-rich melt on Si-based substrates resulted in x-ray rocking curve linewidths as narrow as 72 arc-sec and etch-pit densities in the range 1 to 3 x 10⁶ cm².     

Effect of surface encapsulation and As4 overpressure on Si diffusion and impurity-induced layer disordering in GaAs,AlxGa1-xAs,and AlxGa1-xAs-GaAs quantum well heterostructures

Data are presented demonstrating that the surface encapsulant and the As₄ overpressure strongly affect Si diffusion in GaAs and Al_xGa_1-xAs, and thus are important parameters in impurity-induced layer disordering. Increasing As₄ overpressure results in anincrease in diffusion depth in the case of GaAs, and adecrease in diffusion depth for Al_xGa_1-xAs. In addition, the band-edge exciton is observed in absorption on an Al_xGa_1-xAs-GaAs superlattice that is diffused with Si and is converted to bulk crystal Al_yGa_1-yAs via impurity-induced layer disordering. In contrast, the exciton is not observed in absorption on GaAs diffused with Si in spite of the high degree of compensation. These data indicate that the Si diffusion process, and the properties of the diffused material, are different for GaAs and for Al_xGa_1-xAs-GaAs superlattices converted into uniform Al_yGa_1-yAs (0 ≤y ≤x ≤ 1) via impurity-induced layer disordering with the amphoteric dopant Si.