B. G. Streetman

A new look at impact ionization-Part II: Gain and noise in short avalanche photodiodes

For Part I see R.J. McIntyre, ibid., vol.46, no.8, pp.1623-31 (1999). In Part I, a new theory for impact ionization that utilizes history-dependent ionization coefficients to account for the nonlocal nature of the ionization process has been described. In this paper, we will review this theory and extend it with the assumptions that are implicitly used in both the local-field theory in which the ionization coefficients are functions only of the local electric field and the new one. A systematic study of the noise characteristics of GaAs homojunction avalanche photodiodes with different multiplication layer thicknesses is also presented. It is demonstrated that there is a definite size effect for thin multiplication regions that is not well characterized by the local-field model. The new theory, on the other hand, provides very good fits to the measured gain and noise. The new ionization coefficient model has also been validated by Monte Carlo simulations.

Noise characteristics of thin multiplication region GaAs avalanche photodiodes

It is well known that the gain‐bandwidth product of an avalanche photodiode can be increased by utilizing a thin multiplication region. Previously, measurements of the excess noise factor of InP/InGaAsP/InGaAs avalanche photodiodes with separate absorption and multiplication regions indicated that this approach could also be employed to reduce the multiplication noise. This letter presents a systematic study of the noise characteristics of GaAs homojunction avalanche photodiodes with different multiplication layer thicknesses. It is demonstrated that there is a definite ‘‘size effect’’ for multiplication regions less than approximately 0.5 μm. A good fit to the experimental data has been achieved using a discrete, nonlocalized model for the impact ionization process.

Quantum dot vertical-cavity surface-emitting laser with a dielectric aperture

Data are presented on an oxide-confined vertical-cavity surface-emitting laser that uses a quantum dot active region. The laser is grown by molecular beam epitaxy, with the quantum dot active region formed from a five monolayer deposition of In0.50Ga0.35Al0.15As. Lasing occurs at wavelengths corresponding to quantum dot transitions, with a room temperature pulsed threshold as low as 560 μA for a 7 μm diameter oxide aperture.

Thin multiplication region InAlAs homojunction avalanche photodiodes

Low excess noise in avalanche photodetectors (APDs) is desired for improved sensitivity and high-frequency performance. Gain and noise characteristics are measured for InAlAs p-i-n homojunction APDs that were grown with varying i-region widths on InP by molecular beam epitaxy. The effective ionization ratio ku2009(β/α) determined by noise measurements shows a dependence on multiplication region width, reducing from 0.31 to 0.18 for multiplication region thicknesses of 1600–200 nm. This trend follows previously shown results in AlGaAs-based APDs, which exhibit reduced excess noise due to nonlocal multiplication effects. These results show that this effect is a characteristic of thin avalanche regions and is not a material-specific phenomenon.

The influence of transit-time effects on the optimum design and maximum oscillation frequency of quantum well oscillators

A small-signal analysis of quantum-well oscillators is presented. The analysis includes the transit-time effects associated with a depleted spacer layer outside the quantum well. These transit-time effects are found to dominate device characteristics and to lead to dramatic increases in achievable negative resistance. Closed-form expressions are derived for specific negative resistance and cutoff frequency, and a universal curve relating maximum transit-time negative resistance, quantum-well current-voltage characteristics, and frequency is found. Design considerations to maximize the oscillation frequency threshold are discussed. The analysis also shows that the effective limit on the maximum oscillation frequency of practical quantum-well oscillators is determined by a combination of impedance matching constraints and minimum-achievable contact resistance. >

Effect of an AlAs/GaAs mirror on the spontaneous emission of an InGaAs‐GaAs quantum well

Data are presented demonstrating a strong influence of a closely spaced AlAs/GaAs distributed Bragg reflector on the spontaneous emission characteristics of an InGaAs‐GaAs quantum well. The mirror to quantum well spacings on different crystal samples correspond to optical path lengths of either 1/4, 1/2, or 3/4 of the emission wavelength. The samples are characterized using photoluminescence, electroluminescence, and reflectivity measurements. Spontaneous emission is found to be greatly enhanced for a 1/2 wavelength spacing, while 1/4 and 3/4 spacings suppress the spontaneous emission by a factor of ≳1000.

HIGH-SPEED RESONANT-CAVITY SEPARATE ABSORPTION AND MULTIPLICATION AVALANCHE PHOTODIODES WITH 130 GHZ GAIN-BANDWIDTH PRODUCT

Previously it has been shown that resonant-cavity, separate absorption and multiplication (SAM) avalanche photodiodes (APDs) exhibit high peak external quantum efficiency (∼75%), low dark current, low bias voltage (<15 V), and low multiplication noise (0.2<k<0.3). We describe the frequency response of resonant-cavity AlGaAs/GaAs/InGaAs SAM APDs. A unity-gain bandwidth of 23 GHz and a high gain-bandwidth product of 130 GHz have been achieved.

ZnSe/CaF2 quarter-wave Bragg reflector for the vertical-cavity surface-emitting laser

Data are presented on an electron‐beam evaporated ZnSe/CaF2 distributed Bragg reflector for use on a vertical‐cavity surface‐emitting laser operating at a wavelength ∼0.98 μm. Mirror characteristics are measured using optical transmission and reflectivity for quarter‐wave structures with varying numbers of pairs from one to five. The optical characteristics of the ZnSe/CaF2 quarter‐wave stack is compared to similar structures of electron‐beam evaporated Si/SiO2 reflectors. The ZnSe/CaF2 mirror is found to be superior to the Si/SiO2 mirror in terms of both higher reflectivity and lower optical loss for all structures investigated. Comparison is also made between ZnSe/CaF2 and Si/SiO2 mirrors in the continuous‐wave performance of AlAs‐GaAs‐InGaAs quantum‐well vertical‐cavity surface‐emitting lasers. Superior laser performance is achieved with the ZnSe/CaF2 mirror in terms of threshold current and lasing efficiency.

Photoluminescence studies of pseudomorphic modulation‐doped AlGaAs/InGaAs/GaAs quantum wells

We discuss the photoluminescence (PL) properties of pseudomorphic modulation‐doped Al0.15Ga0.85As/In0.2Ga0.8As/GaAs quantum wells as a function of temperature. At 4.2 K, hole localization influences the PL linewidth; however, at higher temperatures (77 K) the thermal energy of photoexcited holes is sufficiently large to obtain a reliable measure of sheet carrier density from the PL linewidth. Our results also suggest that information about the interface quality can be obtained from an analysis of the PL linewidth at 77 and 4.2 K. The spectra taken from several samples clearly show that the PL transition energy exhibits a free‐carrier density dependence due to band‐gap renormalization and electric field effects.

Investigation of low growth temperature AlGaAs and GaAs using metal–insulator–semiconductor diagnostic structures

GaAs and Al0.3Ga0.7As layers grown by molecular‐beam epitaxy (MBE) at 250u2009°C and incorporated as the gate insulators in metal–insulator–semiconductor (MIS) structures are examined using capacitance–voltage and current–voltage techniques. Samples which are not annealed, and samples annealed for 10 and 20 min under an arsenic ambient at 600u2009°C are examined. MIS structures using material grown at low temperatures without annealing are found to be extremely leaky at room temperature due to a high defect concentration. For unannealed samples and samples annealed for 10 min, a deep level trap is found to be the source of free carriers in the low growth temperature layer. Capacitance–voltage profiling can be accomplished on both structures, indicating the Fermi level is weakly pinned in these structures. For samples annealed for 20 min, however, no modulation of the device capacitance is possible, and the layer exhibits extremely high resistivity. This indicates that the Fermi level is pinned strongly in samples...