219': IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-27, NO. 1 1 , NOVEMBER 1980

is roughly a factor of 3 lower than typical alloyed Au-Ge con- for perfect crystalline materials. Although crystalline quality tac'fs. Thermal degradation studies now in progress show no is relatively insensitive to deposition conditions, analysis by change in contact resistance after 60 h at 35OoC. Preliminary Nomarski optical microscopy has shown that the smoothest, tests of MESFET structures with an n+ Sn-doped layer on a thin layers can be produced by deposition of Silicides near Si-doped implanted n-channel show drain-source burn-out the stoichiometric 1 : 2 Ni to Si ratios, and lower deposition voltages on the order of 30 percent greater than those of com- rates. parable devices with alloyed contacts. Optimization of this process for power FET applications will be discussed.

TP-A.6 The Behavior of Schottky Barriers to GaAs as a Func- tion of Annealing Temperature-H. J. Hovel and C. Lanza, IBM Thomas J. Watson Research Center, Yorktown Heights, NY 10598.

Thlie properties of tungsten, tantalum, molybdenum, alumi- num: and palladium Schottky barriers to GaAs have been in- vestigated as a function of annealing temperature up to 800°C, using capacitance-voltage, current-voltage, and current-voltage- temperature techniques. The barrier height, Richardson con- stant, and diode perfection factor were determined using the thermionic emission relationship. Moly resulted in the best perfection factor and largest barrier height in the as-deposited condbition, but the barrier was reduced and the perfection fac- tor worsened after annealing above 40OoC. Similar behavior was observed for tantalum. The tungsten barrier height was relatively constant at 0.7 eV for annealing up to 500°C, then decreased to 0.55 eV after annealing at 70OoC. The barrier height versus annealing was much more constant for aluminum Scho'ttky barriers, although the perfection factor increased to more than 2.0. Neither Au nor Pd mesas remained rectifying above 40OoC.

In all cases the Richardson constant A * decreased by several orders of magnitude for annealing above 40OoC. There was also a "roll-off" of the I-V characteristics at current densities abov'e: 1-10 A/cm2, as if series-resistance effects were becoming impamtant. These may be manifestations of interfacial layers being formed between the metal and GaAs.

TP-A7 Silicon-Nickel Silicide Heterostructures Grown by Molecular-Beam Epitaxy-J. C. Bean, J. M. Poate, and K. C. R. Chiu' , Bell Laboratories, Murray Hill, NJ 07974.

Molecular Beam Epitaxy (MBE) has produced growth of silicon-nickel silicide heterostructures. The layers grow epi- taxially with near-perfect crystallinity and may thus yield new device structures based on alternating semiconductor and metallic layers.

Nic:kel silicide has been grown on both (1 1 1) and (100) silicon substrates. Single-crystal silicide films can be grown either by near !:toichiometric codeposition of nickel and silicon or by deposition of pure nickel. The MBE deposition temperatures of 600-800°C result in very rapid silicide formation. Crystal- line silicon can then be immediately overgrown by evapora- tion onto the heated Nisiz surface. Crystalline quality has been assessed by Rutherford backscattering and channeling of MeV He ions. Silicide layers, on (1 11 ) Si, have been grown which have channeling Xmin of 4 percent. Silicon layers over- grown on these silicide layers have Xmin of 6 percent. These values should be compared with Xmin of 3 percent expected

'Present address: Hewlett-Packard, Palo Alto, California.

TP-Bl Buried Heterostructure Pbl-,Sn,Te/PbTel-ySe Di- ode Lasers' -Dumrong Kasemset, Shlomo Rotter, and Cifton G. Fonstad, Department of Electrical Engineering and Com- puter Science, Center for Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139.

Buried stripe-mesa heterostructure diode lasers have been successfully fabricated in the lattice-matched system, PbSnTe/ PbTeSe. These lasers, which emit at -10 pm, operate in a fundamental transverse mode with single longitudinal-mode output over a large operating range (to four times threshold) and operate CW to in excess of 120 K. With continued de- velopment, these structures are expected to be capable of even higher temperature operation and higher order mode discrimi- nation in still wider stripe, higher power devices.

It has recently been demonstrated that lattice-matched PbSnTe/PbTeSe DH laser diodes have much lower threshold current densities and higher temperature operation than con- ventional PbSnTe/PbTe structures.2 To reduce the operating current and to achieve good lateral mode control, however, a true stripe geometry is required. A simple stripe contact has been found to be inadequate because of low epi-layer sheet resistance, and a simple etched mesa is also undesirable be- cause the mesa must be very narrow to ensure only funda- mental lateral-mode operation. To reduce the excess leakage and the total threshold current, and to achieve lateral-mode control with as wide a stripe as possible, the buried heterostruc- ture has been developed.

The buried heterostructure lasers are produced by a two- step liquid phase epitaxy technique with growth temperatures 3 0 0 ' C . The substrate is p-type PbSnTe upon which is grown a lattice-matched layer of p-PbTeSe 5 pm thick and the p- PbSnTe active layer, 1.5 pm thick. Stripe mesas 5 pm wide are then etched into the epi-layers with a 5 percent Br in HBr solution using standard photolithographic techniques. After this, a third layer, a 5-pm-thick layer of n-PbTeSe, is grown over the wafer to complete the structure. Broad area contacts are applied and individual devices are separated by cleaving lasers from bars cut on a wire saw.

The CW threshold of these devices is as low as 60 mA at -80 K, which is the lowest value ever reported for lead salt laser at any temperature. Devices have been operated CW to over 120 K.

The light output increases fairly linearly with input current to four times threshold before it begins to saturate. The CW emission spectrum routinely shows a single, lowest order trans- verse mode (as evidenced by far field patterns) for 1.1 Ith < I < 4 I t h , and substantially single-mode output to over 6 Ith. The dramatic improvement in mode quality and control that this represents over conventional structures is the most im- portant impact these buried heterostructure lasers will have on the applications of lead salt lasers in high-resolution gas spectroscopy and heterodyne detection.

DMR-78-00836. Research supported by the National Science Foundation under Grant

2D. Kasemset and C. G. Fonstad, in 1980ZEDM Tech. Dig., p. 130.