Design of quantum well infrared photodetectors

Abstract

QWIPs which respond to normally incident radiation without the need for an optical grating are of particular interest because they can be fabricated with fewer process steps and increased expected yield. An important contribution of this work is the demonstration of the first n-type QWIP (n-QWIP) which showed a significant detectivity of 4x101 cm-[square root of] H-z/Watt without the use of an optical grating. This detectivity corresponds to a conversion efficiency of 4 % or, equivalently, a responsivity of 270 mA/W. This detectivity is significant because it is large enough for focal plane array performance to be limited by the uniformity of processing rather than the size of the single pixel detectivity. An important part of this work was the development of numerically accurate physical models yielding simple analytical expressions for the QWIP leakage current and photocurrent. This physical model yielded analytical expressions for the number of, and the distance over which, carriers are depleted from quantum wells whenever the photocurrent is larger than leakage current. This depletion capacitance is expected to be important at high frequencies, in situations where the photocurrent is much larger than the leakage current, and in QWIPs designed with a small number of quantum wells (as when the quantum efficiency is large or an optical cavity is used). Studies of the microscopic physics of quantum wells are presented to elucidate the physical origin of the intersubband absorption of normally incident radiation. A key result of this work is the derivation within the framework of k p theory of selection rules for the intersubband absorption of normally incident radiation by hole subbands in a p-QWIP (p-doped QWIP) in the absence of an optical grating. It is found that the absorption of normally incident radiation by holes in a p-QWIP in the absence of an optical grating is largest for heavy hole to light hole transitions. The intersubband absorption of normally incident radiation by electrons in an n-QWIP in the absence of an optical grating is found within k p theory to be much smaller than that in a p-QWIP. It is also found that k p theory predicts that uniaxial strain does not have a large effect on the strength or the selection rules of intersubband absorption because the Hamiltonian describing uniaxial strain has the same (tetragonal) symmetry as that describing the confinement of carriers in the quantum wells along the growth direction. Nonuniformity of device parameters across an array of QWIPs is an important issue. High Resolution X-ray Diffraction (HRXRD) was used to measure the layer width variations of QWIPs grown by molecular beam epitaxy. The spread of the measured full-width-half-maxima of superlattice diffraction peaks with the diffraction order was used with Bragg's Law to obtain the measured layer width variation in the growth direction. It was found that the fractional layer width variation was about 2% for three example growths. This layer width variation is consistent with an effusion cell temperature variation of 1°C during growth. A theoretical study has been made of different noise mechanisms which contribute to QWIP performance. A key result found in this work is that when the signal-to-noise ratio (SNR) is limited by either fixed pattern noise or thermal leakage arrival noise, the optimal number of quantum wells for a maximum in the expected QWIP SNR is roughly ... , where ... is the quantum efficiency of a QWIP having only one quantum well. Common QWIP designs used in industry are evaluated. In particular, the commonly used n-QWIP design in which the confinement barriers are comprised of a semiconductor superlattice is considered. This QWIP design is intended to reduce thermionic leakage by pushing the three-dimensional continuum of energy further up in energy by making the miniband transport through the superlattice barrier the means of photocurrent conduction. A Kronig-Penney model presented in this thesis showed that this QWIP design, with a superlattice comprising the QWIP barriers, is expected to have a tunneling leakage which is, at best, commensurate with QWIP barriers which are made of a single semiconductor material but whose band edge is the average value of the band edges of the semiconductors comprising the actual barrier superlattice. The measured thermionic leakage was found to be in good agreement with a model in which the leakage depends exponentially on an activation energy which varies linearly with the applied bias. A deviation of the measured thermionic leakage from the idealized model is proposed as a quantitative measure of the amount of excessive leakage.