Molecular-beam epitaxial growth of graded band-gap quaternary GaxAlyIn1−x−yAs multilayer heterostructures on InP: Application to a novel avalanche photodiode with an ultrahigh ionization ratio

Abstract

We first describe the growth of device quality compositionally graded quaternary GaxAlyIn1−x−yAs on InP by molecular-beam epitaxy (MBE). Device quality material without misfit dislocations were grown to satisfy the lattice matching condition x+y≂0.47 as closely as possible during the growth of the entire graded region. This was accomplished by keeping the In flux constant (constant cell temperature) while changing the Ga and Al cell temperatures simultaneously to keep the total Ga plus Al flux constant while grading x (and consequently y). This was done using a desktop computer and a data acquisition/control system interfaced to cell temperature controllers and shutter actuators. Next we describe growth and operation of a novel p-intrinsic-n type semiconductor (p-i-n) avalanche photodiode (APD) which incorporates such compositionally graded layers in the i region. The i layer contains a multitude of asymmetric wells with one side abrupt and the other graded, each well surrounded by (Al,In)As. This device yielded a very large hole to electron ionization ratio β/α=50 at −12-V bias and 200 Hz. This value for β/α is the highest reported in any III–V compound. Avalanche multiplication occurs when carriers heated by the electric field in the barrier layers impact ionize carriers which are dynamically stored in the wells. Due to the grading of the exit well/barrier interface and under reverse bias condition there is a substantial reduction in the density of dynamically stored electrons and there is negligible multiplication of electrons resulting in a high β/α ratio.