IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 3 5 , NO. 12, DECEMBER 1988 2453

diode (VID), that had five NDR regions. The five tunneling struc- tures wcrc separated from each other by 500-A n+ InGaAs layers which destroyed electron coherence between the tunneling regions such that each resonant-tunneling structure switched sequentially with increasing bias. The room-temperature I-V characteristics of the VID (diameter of 62 p m ) were near-ideal: /,, = 0.88 mA, I , = 0 . 3 5 m A , V p , = ( 1 . 9 + 0 . 7 . i ) V , 1 s i s 5 , w h e r e i d e n o t e s the ith current peak/valley. We have used the VID to demonstrate a multilevel memory element which has five distinct voltage states that can be set by using small current pulses. The VID was also used in a circuit to generate the parity of an 1 I-bit word. [ I ] H . C. Liu and D. D. Coon, Appl. Phys. Let t . , vol. 50, p. 1246, 1987. [2) S . Sen, F. Capasso, A . Cho, and D. Sivco, IEEE Trans. Elecrron De-

(31 A . A . Lakhani and R . C. Potter, submitted for publication. vices, vol. ED-35, p. 2185, 1987.

VB-4 Multiple-Valued Logic Application of a Triple Well Res- onant Tunneling Diode-C. Kusano, T . Tanoue, H. Mizuta, and S . Takahashi, Central Research Laboratory, Hitachi Ltd., Koku- bunji, Tokyo 185, Japan.

A new resonant tunneling diode (RTD) with four potential bar- riers and three quantum wells is proposed and applied to multiple- valued logic devices which are one of the most promising appli- cations of RTD’s [I]. This is the first report of a single diode ex- hibiting significant double negative differential resistance (NDR) characteristics and operating as a triply stable device with a single supply voltage.

A 500-nm Si-doped ( 1 X 10l8 ~ m - ~ ) GaAs buffer layer, a 10- nm GaAs spacer layer, then a triple well structure consisting of undoped AlAs ( 1 nm)/GaAs (6.9 nm)/A126Ga,6As ( 3 nm)/GaAs (5.7 nm)/A1,,Ga76As ( 3 nm)/GaAs ( 11.9 nm)/AlAs ( 1 nm), and finally a 300-nm Si-doped ( 1 x 10l8 cm-’) GaAs layer, were suc- cessively grown on an n+-GaAs substrate by MBE. These struc- tures were determined by numerical simulation.

The device showed significant double NDR between 180 K and room temperature, exhibiting the best characteristics at 219 K; peakhl ley current ratios were 2.8 and 1.4 with the same peak currents of 4 X lo2 A/cm2 for both NDR peaks. With load resis- tance of 100 fl and applied voltage of 1 V, this diode exhibited three stable states at 0.066, 0.158, and 0.249 V. These voltages were in excellent agreement with numerically simulated values. The numerical simulation also showed that the two resonance voltages can be adjusted independently by varying the width of the wells, thus illustrating the advantage of a triple well structure. These re- sults indicate that this new triple well RTD can realize triple valued logic devices with a single supply voltage. Details of the device fabrication and analysis, as well as a new theoretical approach tak- ing random scattering into consideration, will be described. [ 1 1 F. Capasso er ai., IEEE Electron Device Let;. , vol. EDL-8, pp. 297-

299, 1987.

VB-5 Experimental Analysis of Resonant-Tunneling Hot- Electron Transistors Operated at Room Temperature-T. Mori, K. Imamura, H. Ohnishi, Y . Minami, S. Muto, and N. Yokoyama, Fujitsu Limited,, 10-1 Morinosato Wakamiya, Atsugi 243-01, Ja- pan.

This paper reports on the fabrication of AlAs/InGaAs resonant- tunneling hot-electron transistors (RHET’s) operating at room tem- perature, and shows the evidence of intervalley scattering from the r-valley to the L-valleys in the InGaAs base. This paper also shows that the cutoff frequency ( f r ) measured for the RHET reaches 26 GHz and that the resonant-tunneling-barrier response time is esti- mated to be 1.56 ps.

In 1985, we proposed and fabricated a three-terminal resonant-

tunneling device named RHET [ l ] . The device used an Al- GaAs/GaAs/AlGaAs quantum well resonator as a hot-electron in- jector, and exhibited negative transconductance, thus enabling us to use it as a new “functional device.” In 1987, we reported that the RHET using an InGaAs/InAlAs heterostructure has a higher current gain and higher peak-to-valley ratio than the RHET using a GaAs/AlGaAs heterostructure [2]. However, these devices do not work at room temperature because the thermionic emission cur- rent surmounting collector barriers cannot be neglected due to their small barrier heights.

The device used for this study has a resonant-tunneling barrier consisting of a 26.4-A InGaAs layer sandwiched* by two 23.7-A AlAs layers [3], and a collector barrier of 2000-A Ino.5,A10,48As. These were grown on a semi-insulating InP substrate by MBE. The resonant-tunneling barrier exhibited negative differential resistance at room temperature, while the collector barrier is a good electrical isolator at room temperature, thus enabling us to operate the RHET at room temperature. This device uses a 500-A nt-Ino,S3Ga0.47As base, doped to a concentration of 1 x 10l8 ~ 1 1 1 ~ ~ .

The collector current and base current were measured at room temperature as functions of base-emitter voltage with a constant 3 V on the collector in the common-emitter configuration. It has been found that the current gain (and the differential current gain) in- creases with the base-emitter voltage and peaks at a base-emitter voltage of 0.64 V (0.56 V ) . As the base-emitter voltage was in- creased further, the current gain decreased. The peak differential current gain was measured at 2.3. From the fact that the peak volt- ages are about equal to the r - L separation energy, the decreased current gain is considered to be due to the intervalley scattering of electrons from the r-valley to the L-valleys in the InGaAs base.

The scattering parameters of the RHET were measured in a fre- quency range from 0.2 to 20.2 GHz using the collector current density as a parameter. This was then analyzed using the equivalent RHET circuit that we proposed. The cutoff frequency ( fr) was measured at 0.76 GHz at a collector current densi.y of 3.3 X IO2 A/cm2 and 26 GHz at collector current of 3.3 x IO4 A/cm2. De- vice parameters such as the collector capacitance and base resis- tance derived from the equivalent circuit analysis agree well with those estimated by dc measurements or the theory. The high-fre- quency capacitance and conductance of the resonant-tunneling-bar- rier could be determined using this analysis. The capacitance and conductance were determined to be 93.0 f F (emitter area is 1.5 X 20 pm2) and 1.98 mS/pm2 at collector current density of 3.3 X lo4 A/cm2. The resonant-tunneling-barrier response time was de- termined to be l .56 ps.

This work was supported by MITI’s Project of Basic Technology for Future Industries. [ l ] N. Yokoyama et a l . , Japan. J . Appl . Phys., vol. 24, p. L853, 1985. [2] K . Imamura et a / . , Electron. Lett., vol. 23, p. 870, 1987. [3] T. Inata et a l . , Japan. J . Appl. Phys . , vol. 26, p. L1332, 1987.

VB-6 Negative Differential Resistance in AIAs/NiAI/AIAs Metal Base Quantum Wells: Toward a Resonant Tunneling Transistor-N. Tabatabaie, T . Sands, J. P. Harbison, H. L. Gilchrist, and V . G. Keramidas, Bell Communications Research, Red Bank, NJ 07701.

Stoichiometric NiAl is a Hume-Rothery, ;-electron metallic al- loy, and has been studied extensively in the literature. We have recently shown that high-quality epitaxial layers of NiAl can be grown nearly lattice-matched to GaAs substrates. * Furthermore, NiAl films with the proper crystallographic variant provide excel- lent seeding for low defect density semiconductor overgrowth. As the growth technology matures, progressively thinner continuous films are being grown epitaxially, making the GaAs/AIAs/NiAl system ideal for the fabrication of new quantum interference de- vices.

2454 IEEE TRANSACTIONS ON ELECTRON DEVICES. VOL. 35. NO. 12, DECEMBER 1988

We report the observation of a negative differential resistance region (NDRR) in the axial current-voltage characteristics of a nominally 13 monolayer epitaxial NiAl film sandwiched between two 1.2 monolayer AlAs layers. The active area is separated by two 100-A undoped GaAs regions from the heavily doped n-type GaAs electrodes on both sides. The NDRR is observable at room tem- perature in many of the devices but varies in strength across the wafer. The highest documented peak-to-valley ratio at room tem- perature is two, but the majority of the devices exhibit lower ratios, typically around 1.2. The onset of the NDRR occurs around 800 meV. This value, however, contains the voltage drop at the top contact which is believed to be substantial. A true four-terminal measurement of the I-V characteristics is currently being pursued in order to establish the exact NDRR behavior. A second structure containing a 20 monolayer NiAl film also exhibits NDRR at 77 K, the strength of which, however, is substantially weaker than the thiner NiAl structure. The general characteristics of the observed NDRR is remarkably similar to those routinely obtained in our AlAs/GaAs/AlAs double-bamer resonant tunneling structures. On the basis of this similarity, we have identified the origin of this effect to be resonant tunneling through quantized electronic levels is the thin metal layer. To our knowledge, this is the first direct observation of size quantization in semiconductor-metal hetero- structures. The above is further substantiated by the results of our lateral conduction experiments. The conductivity of thin NiAl films is cqmpletely dominated by weak localization for layers as thick as 50 A indicating two-dimensional conduction.

The above system is quite promising for the fabrication of res- onant tunneling transistors for two reasons. First, the band discon- tinuity between the metal and the semiconductor is large. As a re- sult, the tunneling electrons are injected into a high index subband of the thin metal quantum well. The large density of carriers in the occupied lower subbands insure the metallic behavior of the thin films even under axial bias. This facilitates independent control of the potential inside the quantum well, required for transistor ac- tion. Furthermore, we have already demonstrated that the semi- conductor overgrowth can be removed selectively allowing easy access to the thin metal layer for contacting. We are actively pur- suing transistor action in these structures.

*Two talks covering the structural properties and the MBE growth of these buried NiAl films will be presented at the 1988 Electronic Materials Conference.

VB-7 Characterization of p-N Si, - .Ge,/Si Heterojunctions Grown by Limited Reaction Processing-C. A. King, J . L. Hoyt, C. M. Gronet, and J . F. Gibbons, Stanford Electronics Labs, Stan- ford, CA 94305, and M. P. Scott, S. J . Rosner, G. Reid, S. Lad- erman, K. Nauka, and T. I. Kamins, Circuit Technology R & D, Hewlett Packard Co., Palo Alto, CA.

Strained layer growth of Sil-,Ge, alloys on Si has received a great deal of attention because of the rapid decrease in bandgap that results with increasing Ge mole fraction in the alloy. For this reason the %/Sil -,Ge, system is attractive for such device appli- cations as infrared detectors and heterojunction bipolar transistors. The appeal of the Si/Si, rGer materials system is derived in part from its potential process compatibility with existing silicon tech- nology. Because of the structural metastability of the strained layer, however, thicknesses must be kept below a critical value, and ther- mal exposure must be minimized.

In this work, p-N Si, -,Ge,/Si heterojunction diodes were grown by limited reaction processing [ I ] , [2] (LRP) and fabricated into device structures. Alloy layer thicknesses were varied from 60 to 350 nm with x = 0.22. A key objective of this experiment was to investigate the dependence of the forward current ideality factor on layer thickness and its relationship to the onset of dislocation for- mation. In addition, A E g , the bandgap difference between Si and

the Sil _,Ge, alloy layer, can be measured by monitoring the tem- perature dependence of the diode saturation current.

Device fabrication was initiated with the epitaxial growth of an n-type Si layer and a p-type Sil -,Ge, alloy layer on a ( 100 ) n+ Si wafer. Growth of the Sil -,Ge, alloy layer was carried out at a temperature of 625°C using SiCI,H,, GeH,, and B,H, as the Si, Ge, and B sources, respectively. The ratio of SiCI2H2 flow to GeH, flow was 28: 1 for a Ge fraction ot 22 percent. The growth rate under these conditions is about 40 A/min. Mesa diode structures were formed by plasma etching through the top p-type Si, - ,Ge, layer into the underlying n-type Si layer. S i02 was then deposited at a temperature of 380°C in order to passivate the sidewalls. Con- tacts to the diode were formed by evaporating Ti/AI and annealing in forming gas at 350°C for 10 min.

The thicknesses of the p-type Sil - ,Ge, layers for the devices studied in this experiment have values of 60, 80, 90, 100, 110, 220, and 350 nm. The forward current-voltage characteristic of diodes with Si,-,Ge, layer thickness of 100 nm and below have ideality factors n, which are less than 1.01 from turn on to a point where bulk resistance dominates. At a thickness of 110 nm, the low current ideality factor degradation becomes detectable. At 220 nm, n = 1.24, and when the thickness reaches 350 nm, no clear ideal region even exists. The increase in recombination current with thickness is believed to be a result of the generation of electrically active defects in the vicinity of the heterointerface accompanying strain relaxation. Measurements of the diode saturation current ver- sus temperature reveal a strong dependence on the bandgap of the Si, _,Ge, layer. [ I ] J . F. Gibbons, C. M. Gronet, and K. E. Williams, “Limited reaction

processing: Silicon epitaxy,” Appl. Phys. Letr., vol. 47, pp. 721-723, Oct. I , 1985.

[2] C. A. King, C. M. Gronet, and 1. F. Gibbons, “Electrical character- ization of in situ epitaxially grown Si p-n junctions fabricated using limited reaction processing,” IEEE Electron Device Lett . , May 1988.

VIA-1 A Comparative Study of Electroluminescence in Rare Earth (Er, Yb) Doped InP and GaAs Light-Emitting Diodes- Peter S . Whitney, Kunihiko Uwai, Hiroshi Nakagome, and Ken’ichiro Takahei, NTT Basic Research Laboratories, 3-9-1 1 Mi- dori-cho, Musashino-shi, Tokyo 180, Japan.

Recently, rare earth doping of 111-V semiconductors has at- tracted increasing interest for possible application to temperature- stable light sources, such as semiconductor lasers, for optical com- munications. Such devices could combine the advantages of the sharp, stable rare earth luminescence with those of semiconductor minority-camer injection devices. There are to date, however, very few reports on the characteristics of light-emitting devices incor- porating rare earth atoms [1]-[6]. Indeed, very little is known about the exact nature of the emitting centers and the energy transfer mechanisms from the injected carriers to the rare earth ions in these devices.

In order to help clarify these issues, we have made a study of the electroluminescence from erbium-doped InP and GaAs, as well as ytterbium-doped InP light emitting diodes (LED’s) grown by metalorganic chemical vapor deposition (MOCVD). * The charac- teristics of the observed electroluminescence from the internal 4f- 4ftransitions of E?’, at 1.54 pm, and Yb3+, at 1 .OO pm, are re- ported and compared with an emphasis on the temperature depen- dence of the emission intensity. This represents the first report of electroluminescence from Yb : InP grown by MOCVD and the first from Er : InP grown by any method.

In the erbium-doped LED’s the emission at 1.54 pm shows a gradual decline in intensity with increasing temperature up to about 200 K followed by a more rapid decline up to room temperature. The total drop in intensity is about a factor of 20 from 77 K to room temperature. This is in contrast to the case of the ytterbium- doped LED’s, for which the rare earth-related intensity drops