(AlGa)As grown by low pressure metalorganic vapor phase epitaxy using a N2 carrier

We have studied the growth of Al_xGa_1−xAs (0.24<x<0.34) using a N₂ carrier in low pressure metalorganic vapor phase epitaxy. Growth temperature, gas velocity, and V/III ratio were varied to achieve optimum growth conditions. Layers with excellent morphology and electrical and optical properties comparable to samples grown using standard conditions (with a H₂ carrier) can be deposited in a nitrogen ambient. Al_0.24Ga_0.76As bulk material grown on an AlAs buffer layer with a background doping of 1.3×10¹⁶ cm⁻³ showed Hall mobilities of 4500 and 2300 cm²/Vs at 77 and 300K. Photoluminescence studies at 2K revealed strong bound exciton transitions with a full width at half maximum of 5.2 meV for Al_0.29Ga_0.71AS.     

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).     

OMVPE growth of p-AlGaAs/GaAs heterojunctions using diethylberyllium

We report on the OMVPE growth of modulation doped p-type Al_0.43Ga_0.57As(Be)/GaAs heterojunctions which exhibit a two-dimensional hole gas (2DHG). Hole mobilities de-termined by Hall or cyclotron resonance measurements at 300, 77, and 4 K were 394, 3750, and 21200 cm²/V bs s respectively for a sheet carrier density of about 4.5 × 10¹¹ cm^s−2. Beryllium doping of Al_xGa_1−xAs using diethylberyllium is characterized by Hall measurements, secondary ion mass spectrometry, and photoluminescence. The depen-dence of free carrier concentrationvs AlAs% forp ⁺ layers of Al_xGa_1−xAs_x,x = 0–0.5, is determined. A free carrier concentration greater than 1 × 10¹⁸ cm^s−3 is achieved forx = 0–0.43 with no carrier freeze-out down to 77 K.     

A graded-gap detector of ionizing radiation

The current response of Al_xGa_1−x As graded-gap layers to optical and X-ray radiation was studied. A graded-gap electric field in the 15-μm-thick Al_xGa_1−x As layers, with x varying from 0 to 0.4, ensures the complete collection of charges generated by ionizing radiation and makes it possible to attain the value of 0.25 A/W for the current-power sensitivity of Al_xGa_1−x As. In the layers with a lowered doping level of the narrow-gap region of the graded-gap Al_xGa_1−x As layer, the voltage-power sensitivity to X-ray radiation with energy lower than 15 keV is as high as 1.6×10³ V/W in the photovoltaic mode. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 36, No. 1, 2002, pp. 124–128. Original Russian Text Copyright ? 2002 by J. Požela, K. Požela, Šilėnas, Jasutis, Dapkus, Kinduris, Jucienė.     

Electrical and optical characterization studies of lower dose Si-implanted AlxGa1−xN

Electrical and optical activation studies of lower dose Si-implanted Al_xGa_1?xN (x=0.14 and 0.24) have been made systematically as a function of ion dose and anneal temperature. Silicon ions were implanted at 200 keV with doses ranging from 1×10¹³ cm^?2 to 1×10¹⁴ cm^?2 at room temperature. The samples were proximity cap annealed from 1,100°C to 1,350°C with a 500-Å-thick AlN cap in a nitrogen environment. Nearly 100% electrical activation efficiency was obtained for Al_0.24Ga_0.76N implanted with a dose of 1 × 10¹⁴ cm^?2 after annealing at an optimum temperature around 1,300°C, whereas for lower dose (≤5×10¹³ cm^?2) implanted Al_0.24Ga_0.76N samples, the electrical activation efficiencies continue to increase with anneal temperature up through 1,350°C. Seventy-six percent electrical activation efficiency was obtained for Al_0.14Ga_0.86N implanted with a dose of 1 × 10¹⁴ cm^?2 at an optimum anneal temperature of around 1,250°C. The highest mobilities obtained were 89 cm²/Vs and 76 cm²/Vs for the Al_0.14Ga_0.86N and Al_0.24Ga_0.76N, respectively. Consistent with the electrical results, the photoluminescence (PL) intensity of the donor-bound exciton peak increases as the anneal temperature increases from 1,100°C to 1,250°C, indicating an increased implantation damage recovery with anneal temperature.     

Growth of high-quality 1.93-eV AIGaAs using metalorganic chemical vapor deposition

High-quality Al_xGa_1-xAs with a bandgap of 1.93 eV has been grown using metalorganic chemical vapor deposition (MOCVD). Levels exceeding 10₁₈ cm_-3 can be obtained for Se, Si and Zn dopants. In particular, incorporation of the re-type dopants at this bandgap is not appreciably less efficient than in lower-bandgap Al_xGa_1-xAs. The best material, as measured by photoluminescence intensity, is obtained using high V/III ratios (40 forp-type material and as high as 60 forn- type and low growth rates (300 Å/min). Purification of the arsine in situ with an Al/Ga/In eutectic bubbler to remove trace water and oxygen impurities is found to be essential for the growth of high-quality material, as indicated by photoluminescence intensity measurements. Solar cells fabricated from such material exhibit efficiencies as high as 12.1% under one-sun, airmass zero (AMO) conditions, with an open-circuit voltage of 1.38 V, short-circuit current density of 14.1 mA/cm², and fill factor of 0.84.     

Electrical properties of manganese doped Ga1−xInxAs grown by liquid phase epitaxy

Mn-doped Ga_1?xIn_xAs crystals (0 < x < 0.25) have been grown by the LPE technique, and the doping characteristics and electrical properties of the layers have been studied by Hall measurement. The distribution coefficient of Mn has been found to depend on the substrate orientation. The acceptor enerby level is about 77 meV and is comparable to that of Mn-doped GaAs. p-n junction diodes with high InAs compositions, grown using the step grading technique, showed a diode factor of 2. Electron diffusion lengths greater than 3μm have been measured in these Mn doped layers.     

Rectification in AlxGa1-xAs-GaAs N-n heterojunction devices

The previously unsolved problem of rectification at Al_xGa_1-xAs-GaAs N-n heterojunction is found to originate from a vague concept regarding the maximum junction grading width which can sustain rectification. The theoretical current density vs voltage characteristics of this heterojunction system are derived from thermionic emission theory. It is found that, unless the impurity concentration of the AlGaAs layer (prepared by LPE techniques) is less than 10¹⁶ cm^?3, typical 90–200 Å metallurgical grading widths at the N-n heterojunction interface produce either ohmic or poorly rectifying characteristics. These results explain (1) the lack of rectification in most N-n Al_xGa_1-xAs-GaAs heterojunctions reported in the literature and (2) the recent observation of significant rectification in high purity (N)Al_0.3Ga_0.7As-(n)GaAs heterojunctions reported by Chandra and Eastman.     

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.     

Effects of high doping on the bandgap bowing for AlxGa1−xN

This paper gives the composition dependence of the bandgap energy for highly doped n-type Al_xGa_1−xN. We report results of the bowing parameter obtained using a random simulation. Three groups of Al_xGa_1−xN semiconductors were considered and which are distinguishable by their non degenerate or degenerate character in the doping density (10¹⁷?N_D?10²⁰ cm⁻³). A striking feature is the large discrepancy of the bandgap bowing (−2.02?b?2.94 eV), as was demonstrated from our calculations. This suggests that high doping may be a possible cause able to induce the large range of bowing parameters reported for Al_xGa_1−xN alloys.     

Layered Growth of Lattice-Mismatched Ga x In1−x P on GaP Substrates by Liquid Phase Epitaxy

We report layered growth of Ga _x In_1?x P on GaP substrates using single-step liquid phase epitaxy (LPE) with a Sn-based melt when the lattice mismatch is greater than 0.4 % (x < 0.95). Compositional control was observed by (1) varying the cooling rate and (2) changing the melt-back temperature at the beginning of the growth. Possible growth mechanisms are proposed to explain the principles of both approaches of compositional control. Smooth epilayers have been observed. High resolution x-ray diffraction was used to characterize the composition of the epilayers, and room temperature photoluminescence was reported for one of the samples with the composition of x = 0.11. Plan-view TEM measurements revealed threading dislocation densities on the order of 10⁸ cm^?2 in the upper regions of the Ga _x In_1?x P epilayers. In contrast, when using In-based melts, LPE of Ga _x In_1?x P on GaP (100) substrates exhibited island growth at large misfits, whereas edge growth dominated when using GaP (111B) substrates under equivalent growth conditions.     

Electronic structural study of liquid phase epitaxy grown AlxGa1−xAs:Be using temperature dependent (≥ 10 K) laser excited photoluminescence

Crystals of Al_0.26Ga_0.74As doPed witn Be were grown by liquid phase epitaxy (LPE), and their laser excited photoluminescence (PL) spectra were accurately explored in the bandgap to deep level energy range. The temperature dependent (10 K ≤ T ≤ 70 K) and laser excitation power dependent PL data revealed that the 1.808 eV recombination is of D-A origin. From the thermal quenching and other experiments a 13.9 meV activation energy was found and is believed to be D_Si. From the data about the 1.808 eV recombination and the one at 1.841 eV, it was also determined that the activation energy is 38 meV for A_Be. No radiative deep level defects were found to be present in these LPE AlGaAs:Be crystals. The unintentionally introduced donor creates D-A recombinations competitive with CB-A recombinations and suggest the exercise of greater care in LPE growth of AlGaAs.     

Growth and evaluation of lpe graded composition AlxGa1−xAs layers for high efficiency graded bandgap solar cells

A melt-mixing LPE growth technique to obtain a graded composition Al_xGa_1−xAs layer is described. The graded bandgap AlGaAs/GaAs solar cell requires the Al fraction (x) in the Al_xGa_1−xAs surface layer to increase from the junction towards the surface. The graded composition Al_xGa_1−xAs layer creates a built-in electric field for minority carriers which improves the carrier collection process. This graded bandgap solar cell should enable one to realize the full potential of GaAs as a material for solar energy conversion. Quantitative evaluation of the composition profile for these graded layers is needed to develop the proper growth technique and to understand the resulting solar cell characteristics. Rutherford backscattering analysis technique is described which has been successfully used to profile such layers.     

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.     

Ge-doped InxGa1−xAs p−n junctions

The amphoteric properties of Ge in the In_xGa_1?xAscrystals grown by liquid-phase epitaxy are reported. It was found from the Hall measurements that when x < 0·1, the Ge-doped In_xGa_1?xAs was p-type, and when x > 0.1,it was n-type . At the vicinity of x = 0.1, therefore, the In_xGa_1?xAs p?n junction could be made by one growth process. The electrical and photoelectric characteristics of that junction were investigated. The distribution of Ge concentration at the p?n junction, which was obtained from the C-V characteristics, depended on the doping concentrations of Ge. This dependence can be interpreted by analyzing a modified Longini-Green equation. The spectral responses of both photovoltaic effect and electroluminescence showed that in In_0.1Ga_0.9As, Ge atoms gave rise to a heavy compensation effect, and introduced a conduction band tail of states and two kinds of acceptor levels located ～ 20and～ 100meV above the valence band edge.     

In0.2Ga0.8As/GaAs quantum well laser with C doped cladding and ohmic contact layers

Carbon doping in Al_xGa_1−xAs was achieved using different approaches. The moderate growth temperature of 650°C was employed to grow C bulk-doped Al_xGa_1−xAs with a high Al mole fraction. The hole-density was altered using different V/III ratios. The trimethylaluminum (TMAl) was used as an effective C δ-doping precursor for growth of C δ-doped pipi doping superlattices in Al_xGa_1−xAs. the average hole-density of C δ-doped pipi superlattices was greater than 2−3 × 10¹⁹ cm⁻³. Zn-free GRINSCH In_0.2Ga_0.8As/GaAs laser structures were then grown using the C bulk-doped Al_xGa_1−xAs and C δ-doped pipi superlattice as a cladding and ohmic contact layer, respectively. The ridge waveguide laser diodes were fabricated and characterized to verify flexibility of these two doping approaches for device structures.     

Thermoelectric Properties of Ga-Doped Ba8Al x Si46−x Clathrate

Single-crystalline Ba₈Al _x Ga _y Si_46?x?y clathrates were synthesized by the arc melting method and Czochralski method without subsequent treatment, and their thermoelectric properties were compared with those of Ba₈Al _x Ga _y Si_46?x?y and Ba₈Al _x Si_46?x clathrates with almost the same carrier concentration as estimated from the similar Seebeck coefficient and the Zintl concept. The resistivity of Ba_7.8Al_5.3Ga_7.4Si_33.3 was lower than that of Ba_7.9Al_12.6Si_33.4. The specific electrical resistance of Ba_7.9Al_12.6Si_33.4 and Ba_7.8Al_5.3Ga_7.4Si_33.3 was 0.573 mΩ cm and 0.282 mΩ cm at 750 K, respectively. The band structure of Ba₈Al₈Ga₈Si₃₀ and Ba₈Al₁₆Si₃₀ was estimated by first-principle calculations using density functional theory with the local density approximation. Based on these calculations, it was found that the shape of the bottom of the conduction band for Ba₈Al _x Si_46?x clathrate changed slightly on Ga doping and the radius of curvature of the bottom of the conduction band for Ba₈Al₈Ga₈Si₃₀ clathrate was lower than that for Ba₈Al₁₆Si₃₀ clathrate. These results indicate that the mobility was enhanced by Ga doping of Ba₈Al _x Si_46?x clathrate. We also synthesized single-crystalline Ga-doped Ba₈Al _x Si_46?x clathrate. The electrical resistivity decreased dramatically due to the single-crystallization because of reduced electron scattering on grain boundaries. These results suggest that Ga doping and single-crystallization are effective for improvement of the thermoelectric properties of Ba₈Al _x Si_46?x clathrate.     

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.     

Classical magnetoresistance measurements in AlxGa1−xAs/GaAs MODFET structures: Determination of mobilities

Classical magnetoresistance techniques have proven to be quite useful for obtaining mobility profiles in GaAs MESFET structures. Here we extend these techniques to Al_xGa_1−xAs/GaAs MODFET structures, which are more complicated because of multi-band conduction effects. A multi-band geometric-magnetoresistance (GMR) theory is developed in the relaxation-time approximation, and average and differential mobility expressions are defined. The magnetic-field dependences of these quantities permit determination of the mobilities associated with the various conducting bands. We apply this analysis to an Al_0.3Ga_0.7As/GaAs MODFET structure and obtain results for μ₀, the lowest subband mobility, μ_1–3, a combination of higher-subband mobilities, and μ_AlG, the mobility in the heavily doped Al_0.3Ga_0.7As layer. Complicating factors such as finite Hall field, energy-dependent relaxation time, parasitic resistance and gate current are also considered.