Semiconductor Superlattice Infrared Source

The present invention relates to a semiconductor device with an infrared emitting active region which consists of a layered structure of semiconductor material formed of a combination of thin layers having a superlattice structure.

U.S. Pat. No. 4,644,378 describes one type of light-emitting semiconductor device which produces radiation in the visible and near-infrared portion of the spectrum. This device has a light-emitting region formed by a layered structure with very thin active layers of a first semiconductor material located between and separated by very thin barrier layers of a second semiconductor material which has a larger energy gap than that of the first semiconductor material. These alternating very thin layers form a superlattice structure. In this device, the active layers are all of equal thickness having a thickness of n monolayers. The barrier layers are also of equal thickness having a thickness of m monolayers where n and m are integers between 2 and 7 with the value of n+m.ltoreq.12. The maximum thickness of 7 monolayers is that at which the active layers are still sufficiently thin to exhibit a strong quantum well effect, while the barrier layers are thin enough to produce a pronounced zone folding effect. The active layers in this device may be formed of GaAs with the barrier layers consisting of AlAs.

U.S. Pat. No. 4,630,083 describes another type of light-emitting semiconductor device with a laminated active layer consisting of five GaAs layers about 10 nm thick and four AlGaAs layers about 10 nm thick, where the GaAs layers are separated from each other by one of the AlGaAs layers. The AlGaAs layers have a larger band gap than the GaAs layers providing a quantum well structure i.e. a superlattice structure. The GaAs ultrathin layers service as quantum wells and the AlGaAs ultrathin layers serve as barriers. Both U.S. Pat. Nos. 4,644,378 and 4,630,083 deal with transitions between conduction and valence bands in superlattice structures.

Another type of device is described in Applied Physics Letter, Vol. 51 of Sep. 21, 1987 on page 934. This device is a semiconductor photodetector with a superlattice structure, in particular a 10 .mu.m radiation detector. This detector consists of 50 periods of 70 .ANG. GaAs quantum wells separated by 133 .ANG. Al.sub.0.36 Ga.sub.0.64 As undoped barriers. In this device, an electron in the ground state of the quantum well can be raised to the first excited state by photoexcitation resulting in an intersubband transition in that layer. Under proper bias this electron can tunnel out of that layer providing a photon generated current. Another 10 .mu.m infrared detector is described in Physical Review Letters, Vol. 59, No. 21, Nov. 23, 1987. This last-mentioned device provides a structure where coherent tunneling of photoexcited electrons through a double-barrier superlattice structure can occur. Each period in this laminated structure consists of a 72 .ANG. thicker well of GaAs (doped), a 39 .ANG. thinner barrier of undoped Al.sub.0.33 Ga.sub.0.67 As, a 18 .ANG. thinner well of undoped GaAs and a 154 .ANG. thicker barrier of undoped Al.sub.0.33 Ga.sub.0.67 As. The wider uncoupled wells have two states E.sub.1 =47 meV and E.sub.2 =183 meV in the energy-band diagram whereas the narrow well has only one state E'.sub.1 =174 meV. Under forward bias where E'.sub.1 is approximately aligned with E.sub.2 of the adjacent period, the coherent tunneling of a photoexcited electron from the wide well through the state E'.sub.1 of the narrow well and out of the barriers is possible. These two articles are directed to infrared detectors having a superlattice structure.

An object of the present invention is to provide a new type of infrared emitting semiconductor device which emits in the mid-to-far infrared range (2 to 10 .mu.m or longer) with a narrow emission spectrum where the wavelength which is emitted can be adjusted by varying the device parameters.

A further object of the invention is to provide an improved infrared emitter having a novel superlattice structure which utilizes intersubband transitions and resonant tunneling phenomena through double-barrier structure to generate infrared radiation.

A still further object of the present invention is to provide an infrared emitter with a special superlattice structure which gives several improvement factors to make the photoemission process favourable and efficient.

A preferred embodiment of the invention comprises an infrared-emitting semiconductor device with an active region consisting of alternating thin layers of semiconductive materials forming a finite superlattice structure wherein the structure consists of alternating narrow and wide wells separated by thin barriers in which the narrow well has only one quasibound state E'.sub.0 while the wide well has two states E.sub.0 and E.sub.1.

In a further preferred embodiment, the superlattice structure is formed of Al.sub.x Ga.sub.l-x As-GaAs material with x being less than or equal to 1.

The invention will now be in more detail with reference to the accompanying drawings, wherein:

FIG. 1 is a cross-sectional view of an edge of an infrared emitting semiconductor device according to the present invention,

FIG. 2 is an energy-band diagram of the structure under no bias and

FIG. 3 is an energy-band diagram of the structure under sufficient bias to emit infrared radiation.

The layered structure of a semiconductor device according to the present invention is shown in FIG. 1. The layers in FIG. 1 constitute a superlattice structure having thin narrow wells 1 and wide wells 2 separated by barrier layers 3. The structure can be formed of AlGaAs-GaAs material AlGaAs for barriers and GaAs for wells. Both semiconductive materials are undoped (or not intentionally doped) to be substantially electrically nonconducting. The narrow well has a width w' as shown in FIG. 2 and only one quasibound state E'.sub.0. The wide well 2 has a width w and has two states E.sub.0 and E.sub.1 where E.sub.0 is the ground state and E.sub.1 is the first excited state. The energy-band diagram of the device is shown in FIG. 2 under 0 bias with E'.sub.0 .apprxeq.(E.sub.0 +E.sub.1)/2 by adjusting the well widths, w' and w. Heavily N-type doped contact layers 4 and 4' are provided at the top and bottom of the superlattice structure as shown in FIG. 1 so that photon fields are confined in the undoped superlattice structure.

FIG. 3 illustrates the energy-band diagram of the device when it is biased into operation. Under operating conditions, electrons are injected into the higher state E.sub.1 of the wide well 2 by resonant tunneling through E.sub.0 ' of the narrow well. This type of injection process is efficient with the tunneling probability being close to one. An electron in the higher state E.sub.1 of a wide well 2 can relax to a lower state E.sub.0 emitting a photon (h.upsilon.) in the process. This relaxed electron at E.sub.0 continues to tunnel resonantly to the next wide well where it is again located at the higher state E.sub.1 and relaxes to the lower E.sub.0 emitting another photon. This process continues from wide wells to wide wells providing an infrared emitting device.

The width b of the barrier layers in the device are adjusted to be thin enough so that the tunneling time through a double-barrier structure between two adjacent wide wells is somewhat faster than the radiative relaxation time. For an AlGaAs-GaAs structure with about a 10 .mu.m separation between the ground state and the first excited state subbands, a radiative relaxation time faster than about 0.1 ps may be required. Compared with simple superlattice structures, the present device provides efficient and rapid electron transfers between the active regions formed by the wide wells 2. This favours the radiative emission processes.

In a particular example of a 10 .mu.m infrared emitting device formed of Al.sub.x Ga.sub.1-x As-GaAs material, a choice of an alloy where x=0.45 gives a barrier height of 0.31 eV. A width w=82 .ANG. provides an E.sub.0 =0.042 eV and E.sub.1 =0.166 eV with a width w'=40 .ANG. providing a single state E'.sub.0 =0.103 eV in the narrow well so that E'.sub.0 .apprxeq.(E.sub.1 +E.sub.0)/2. A barrier width b of about 15 .ANG. permits a tunneling time through the double barrier between two adjacent wide wells of less than 0.05 ps. Since the emitted radiation h.nu. has a wavelength of hc/(E.sub.1 -E.sub.0), this particular device will emit radiation in the 10 .mu.m range. The wavelength of the emitted radiation can be varied between about 2 to 10 .mu.m and longer by varying the parameters of the structure.

Various modifications may be made to the preferred embodiment without departing from the spirit and scope of the invention as defined in the appended claims.