antimonide-based resonant tunneling structures with ternary alloy layers

Solid-State Electronics Vol. 37, Nos 4--6, pp. 981-985, 1994

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Copyright ,~' 1994 ElsevierScienceLtd Printed in Great Britain. All rights reserved 0038-1101/94 $6.00+ 0.00

Pergamon

InAs/ANTIMONIDE-BASED R E S O N A N T T U N N E L I N G S T R U C T U R E S WITH T E R N A R Y ALLOY LAYERS J. N. SCHULMAN,D. H. CHOW and T. C. HASENBERG Hughes Research Laboratories, 3011 Malibu Canyon Road, Malibu, CA 90265, U.S.A. Abstract---We have studied two modifications of mixed arsenide/antimonide resonant tunneling structures, in which one of the binary components is replaced by a ternary alloy. The first structure is based on an InAs/AISb/GaSb resonant interband tunneling structure with the GaSb quantum well layer replaced by a GaAs,Sb t _,. alloy layer. The added As decreases the lattice constant in the well, and the resulting strain can shift the heavy hole subbands below the incoming electron energy. We find that the negative resistance effect can be enhanced by properly adjusting the splitting via the As concentration. A maximum enhancement in the peak-to-valley ratio was obtained with a 10% As concentration in the diodes we fabricated. The second structure is a modification of a conventional lnAs/AlSb double barrier structure, in which the AISb barriers are replaced by A1,.Ga~_xSb with x ~ 0.5. In these structures, the pronounced type II band alignment results in an entirely new type of current vs voltage characteristic exhibiting intrinsic current bistability (double-valued currents for a given voltage) instead of the conventional voltage bistability. Peak-to-valley voltage ratios as high as 1.5 have been observed with current densities in the 105 A/cm 2 range.

INTRODUCTION Resonant tunneling structures based on the InAs/ AISb/GaSb material system have produced diodes with unique characteristics and enhanced performance. Two systems have been studied in detail in recent years. First is the interband tunneling structure, which utilizes the broken-gap band line-up of InAs and GaSb, usually in combination with AISb barrier layers[I-3]. In these structures band gap blocking for bias voltages above the tunneling resonance results in extremely high peak-to-valley current ratios. The second structure of current interest is the relatively conventional InAs/AISb double barrier structure. This structure has demonstrated the highest oscillation (700 G H z ) and switching speeds (i.7 ps) to date for resonant tunneling devices[4, 5]. The individual layers in both of these devices are usually made from binary compounds, i.e. InAs, GaSb, and AISb. Here we investigate the effects of replacing one or more of these layers with ternary alloys, GaAs~Sb~ _, for GaSb in the case o f i n t e r b a n d structures, and AI,Ga~_ ~Sb for AISb in the case of conventional double barrier structures. We demonstrate dramatic variations in both cases, including an improved peak-to-valley ratio in the first case, and a novel bistable current feature in the second.

InAs]AISb/GaAsxSb I_. INTERBANDTUNNELING STRUCTURE

Figure l(a) is a band edge diagram of a typical lnAs/AISb/GaSb structure. In these devices, electrons are emitted from the lnAs conduction band, tunnel ssE 37 , ~ - D D

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through the AISb barrier into the G a S b valence band resonances, and then out the other side. Other variations which have been studied include reversing the roles of the InAs and GaSb as the cladding and well layers, and making various asymmetric structures[6--8]. Here we are only concerned with cases in which the quantum well portion is in the valence band, as in Fig. l(a). Shown schematically is the band edge diagram of the structure and two of the valence band resonances, the first heavy hole (HH 1) and first light hole ( L H I ) levels. The H H I level lies above the LHI level, as is always the case for unstrained materials. The close proximity of H H I above L H i produces a degradation in the negative differential resistance effect, however, which can be understood as follows. The LHI state is the dominant channel for tunneling due to the large coupling between the lnAs conduction states and the G a S b light hole state[9]. The coupling to the heavy hole state is small due to the differing orbital composition of the heavy hole states near the Brillouin zone center. As bias is applied to the diode, the incoming electrons will encounter the H H i resonance at higher biases than the LHI resonance. Thus, depending on the precise widths of the layers and other structural parameters, tunneling through the HH1 level contributes to the valley current and tends to degrade the peak-to-valley current ratio of the device. In addition, if the well layer is thick enough, there may be one or two additional heavy hole levels above the LH I level contributing to the valley current. The valley current can be divided into two contributions, that due to scattered and due to unscattered

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J.N. SCHULMANet al.

electrons. Theoretical calculations usually include only the unscattered portion[9, 10]. This calculated heavy hole current contribution to the valley current is small and is probably undetectable, especially at higher temperatures. The main contribution, however, is probably the more difficult to account for contribution due to scattering. This is certainly the case for conventional structures such as n-type GaAs/GaA1As double barrier diodes. In this case, experimental values for the valley currents are usually much larger than the theoretical predictions. The major missing theoretical ingredient is generally agreed to be the scattering through the indirect conduction band valleys, via phonons, impurities, defects, or other mechanisms. It would be reasonable to expect that similar mechanisms involving scattering through the heavy hole band are significant in the InAs/A1Sb/GaSb system. We have attempted to eliminate the heavy hole channel by adding As to the central GaSb well layers. Since GaAs has a smaller lattice constant than GaSb, the introduced As creates a lattice mismatch which splits the heavy hole band to lower energy than the light hole band. Figure l(b) shows a schematic diagram of such a structure. If the mismatch is sufficient, this splitting can dominate over the quantum confinement induced splitting and actually push the heavy hole band below both the light hole level and the InAs conduction band minimum energy. It is then entirely inaccessible for tunneling at any applied bias. We have grown by MBE and fabricated InAs/ AISb/GaAsSb structures with As concentrations of 0, 10, 15 and 30%. The arsenic concentration in the GaAsxSbl_~ layer was controlled by utilizing a valved arsenic cracking effusion cell. The well and barrier thicknesses were 51 and 12/~, respectively, with the lnAs electrodes doped to 2 x 10tBcm-3, separated from the AISb barriers on either side by lightly doped 600 A InAs spacer layers. Figure 2 shows the room temperature current-voltage curve for one such device, with x = 0.1 in the GaAsxSb~ _~. Table I summarizes the average peak-to-valley ratios and the peak currents in the four structures. Due to variations in device area produced by the processing procedure, the differences among the values of the peak currents for the 0%, 10% and 15% As samples are not large enough to be significant, and

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V

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Fig. 1. Band edge d i a g r a m o f (a) I n A s / A I S b / G a S b a n d (b) l n A s / A I S b / G a A s S b d o u b l e barrier r e s o n a n t tunneling structures.

E

-0.8

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0 V

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Fig. 2. Room temperature current vs voltage curve for InAs/A1Sb/GaAsSbdouble barrier structure with 10% As in the GaAsSb.

should be considered as roughly similar. The peak-tovalley ratios do not depend on area, and it can be seen that the 10% As sample has the maximum peak-tovalley ratio among the set. We believe that this enhanced peak-to-valley ratio is due to the lowering of the heavy hole resonance energy below the light hole resonance energy for this alloy concentration. Calculations of the energy levels in these structures were done using a twenty-band tight-binding model, to include the heavy hole-light hole coupling[10]. The standard offset of 150meV was used for the offset between the InAs conduction band and the GaSb valence band. A valence band discontinuity of 43% of the band gap difference was used between AISb and GaSb to set the A1Sb band edges[3]. These values are relatively well determined in the literature. A much more uncertain parameter is the dependence of the band offsets on the As concentration in the GaAsSb layers. In lieu of any better technique to determine this value, a constant 43% discontinuity rule for the unstrained GaAsSb valence band offset was used. This is somewhat arbitrary, since the accuracy of this procedure is unknown, and therefore the theoretical results should be taken as qualitative only. The calculation for the 0% As case gives a HH1 energy about 60 meV above the LHI energy. Adding 10% As brings the LHI energy to within 15 meV of HHI, but still above it. An additional 10% As, to 20% total, shifts HHI to 35 meV below LHI, which is about 10meV above the lnAs conduction band

Table I. Room temperature peak-to-valley ratios (P/V) and approximate current densities for InAs/AISb/GaAsSb double barrier structures with 0, 10, 15 and 30% As concentrations x

P/V

Peak current density (A/cm:)

0 0.1 0.15 0.30

8 I1 3 3

2.5 × 1 0 4 1,5 × 104 1.3 × 104 0.5 x 104

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Resonant tunneling structures edge. The experimental result of an enhanced peakto-valley ratio for 10% As implies that the HH1 energy should have already passed to below the LH 1 level for this concentration, but given the aforementioned uncertainties, this level of agreement is all that can be expected. We believe that the abrupt decrease in the measured peak-to-valley ratio for 15% and 30% As is due to the composition induced shift of the light hole resonance itself towards the InAs conduction band edge. As LH1 decreases in energy, the overlap of its density of states with the incoming InAs density of states eventually decreases, resulting in smaller currents and degraded peak-to-valley ratios. Thus, we have shown that adding As to the central GaSb layers has a significant effect on the currentvoltage characteristic. Similar effects should be found in related structures, e.g. InAs/AISb/GaSb/AISb/ GaSb and other structures.

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The starting point for the second system to be discussed, InAs/AISb double barrier heterostructures, utilize relatively conventional conduction band resonant tunneling. Their main advantages arise from the use of high mobility InAs in the cladding layers, which provides low contact resistance. The AISb barriers have large conduction band offsets with InAs and are nearly lattice matched. These structures have been used to make oscillators with frequencies up to 712 GHz[4] and switches with 10-90% rise times as short as 1.7 ps[5]. As in the previously described InAs/AISb/GaSb structures, the devices reported here are also based on a type II resonant tunneling structure. However, in this case the staggered lineup involves the barrier layers, instead of the wells. The operating principle of the device is that, under certain conditions, two different steady state charge distributions are obtainable for a single bias voltage. These conditions arise due to the presence of hole states in the quantum barrier layers, providing regions for positive charge to be trapped. As device current is strongly affected (via band bending) by changes in the steady state charge distribution, a pronounced bistability in device current is observed. In all of the structures studied here, the active device region consists of an InAs(n)/ AlxGal_xSb/InAs/ALGal_xSb/InAs(n) stack in which the middle three layers form a double barrier structure for electrons tunneling from one lnAs(n) electrode to the other. Figure 3(a) shows a schematic band edge diagram of this structure at zero bias. The cladding layers consist of ~ l / t m n ÷InAs. Lightly doped (500.~,, n = l x l017cm -3) and undoped (100,~) lnAs spacer layers are used to bracket the active double barrier region. The InAs quantum well layer is 75/~ thick in all samples, while barrier layer compositions and thicknesses are intentionally varied

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lnAs/Al~Gao_xSb CURRENT CONTROLLED NEGATIVE DIFFERENTIAL RESISTANCE STRUCTURE

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__~__ Fig. 3. Schematic band edge diagram of the InAs/ AIxGa~_xSb structure with three potential distributions. (a) Has no applied bias, and (b) and (c) have the same applied bias. Holes may accumulate in the AleGa,_ ~Sb barriers for sufficient bias. from sample to sample. Each structure is completed by a 2000 A InAs(n *) cap layer. The I - V curves are taken on an HP4145A Semiconductor Parameter Analyzer configured either to input a current sweep while measuring voltage, or to input a voltage sweep while measuring current. Due to the nominally symmetric layer sequence used for the structures investigated here, we observe nearly symmetric I - V curves in all of the devices studied. Figure 4(a) is an l - V c u r v e displayed by a structure with symmetric, 18,~ thick (AISb)]ML(GaSb)jML ordered alloy barriers, taken by sweeping the voltage from 0mV up to 750mV and then back down to 0mV, while measuring the current through the device. A pronounced hysteresis is displayed in the figure, with the lower current line corresponding to the sweep from low to high bias. The observation of hysteresis in the d.c. I - V curve is indicative of a bistability in device current for a given applied bias. In contrast, no hysteresis is observed in the I - V curve obtained from a structure with symmetric, 18 A thick AISb barriers (a "conventional" InAs/AISb resonant tunneling diode), taken using the same measurement configuration. Figure 4(b) is the current-voltage curve obtained from the device used for Fig. 4(a) taken by sweeping the current while the voltage is measured. The curve displays a current-controlled ("S"-shaped) negative differential resistance region, in contrast to the voltage-controlled ("N"-shaped) curves displayed by

984

J.N.

SCHULMAN et al.

conventional resonant tunneling diodes. We note that in this measurement configuration there is no voltage bistability (the voltage as a function of current is a single valued function), and that conventional resonant tunneling diodes will normally display hysteresis when tested under this measurement configuration. We have also observed the I - V behavior displayed in Fig. 4(a) and (b) in samples with symmetric: (i) 21 A, thick Al0.4Gao.6Sb barriers, (ii) 15/~ thick Al0.rGa0.4Sb barriers, and (iii) 18 A, thick Al0.sGa0.sSb barriers. At 400 mV applied bias, the current ratio between the high and low states of the I - V curve displayed in Fig. 4 is greater than 2. This figure of merit might be expected to have implications for state separation in digital circuit applications. A potentially more significant figure of merit is the current density obtained from the device, which is greater than 2 × 105A/cm2 in the current-controlled negative differential resistance region of the curve. Large current densities in these devices generally indicate potential for high speed operation due to short r.c. time constants[l l]. The current densities observed here are only a factor of 1.5 smaller than the peak current density of an InAs/AlSb resonant tunneling diode that has demonstrated 1.7ps switching times[5]. Therefore, we anticipate that the class of

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Fig. 4. (a) I - V curve, taken by sweeping the voltage in both directions while measuring current, for an lnAs/ Al0.sGao5Sb double barrier structure with 18 A, thick barrier layers. (b) I - V curve taken by sweeping the current in both directions while measuring voltage. Room temperature.

devices reported here will be suitable for future high speed circuits. The special characteristics of this double barrier structure that produce the unique double-valued current behavior can be understood by considering the band edge diagrams shown in Fig. 3. Figure 3(a) shows the bands with no applied bias. The conduction band edges are similar to those of a conventional double barrier structure. However, the Al~Ga~_xSb layers are barriers for electrons only; they form wells for the holes, with the InAs layers acting as barriers. These hole wells can trap positive charge, depending on the applied bias and the Fermi level in the positively biased side, and significantly alter the potential distribution in the structure. For small bias the current can still be described by conventional conduction band double barrier resonant tunneling. A key point here is that the voltage drop in the space charge region adjacent to the barriers on the positively biased side can be substantial, depending on the doping concentration, and is a large contribution to the total measured voltage. For sufficiently large applied bias the Fermi level on the right approaches the allowed hole energy levels in the right AlxGa~ _.,Sb well. At some point, depending on bias and tunneling rates, the hole population in the AI~Ga~_ xSb layers will increase to the extent that the holes substantially screen out the electric field in the space charge region. This occurs near 0.6 V in Fig. 4. The potential drop will then become greater across the double barriers themselves and smaller in the space charge region. Since the tunneling current follows the voltage across the barriers, this causes the sudden jump in current at that voltage. Figure 3(b) and (c) show band diagrams for the two possible potential distributions for a given total bias. The current is relatively small in (b) due to the smaller voltage drop across the double barriers, while the current is larger in (c) due to the larger voltage drop there. Measurements at low temperature reveal that the conventional negative differential resistance effect due to tunneling through the conduction band resonance also occurs, but at higher bias. In summary, we have designed the demonstrated a novel resonant tunneling device based on type-II, InAs/AI~GaL_ ~Sb heterostructures. Due to hole accumulation in the active device layers, these structures are capable of possessing multi-state charge distributions at a given applied bias, resulting in bistable device currents. The demonstrated devices display hysteretic I - V behavior under voltage-controlled measurement conditions, and "S"-shaped negative differential resistance under current-controlled conditions. Device operating currents are in excess of 1.5 × 105 A/cm-', suggesting the potential for good high speed performance due to low r.c. time constants. Acknowledgements--The authors wish to acknowledge helpful technical discussions with A. T. Hunter, E. T. Croke

Resonant tunneling structures and R. H. Miles and the technical assistance of L. D. Warren.

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