RU2301486C2 - Semiconductor injection laser - Google Patents

Abstract

FIELD: semiconductor devices; lasers built around multipass p-n heterostructures.

SUBSTANCE: proposed laser has doped active region in the form of semiconductor layer whose thickness ranges between 0.1 μm and several diffusion lengths of minor media. Active region is disposed between two wide-band n and p ohmic-contact layers. Additional semiconductor layer, 20 to 500 angstrom thick (quantum well), is grown between wide-band layers, its thickness being smaller by 0.1 kT to 300 MeV than that of active region and thickness of the latter equals minimum two Debye lengths.

EFFECT: improved design.

3 cl, 2 dwg

Description

The invention relates to semiconductor devices, and more particularly to lasers based on multipass pn heterostructures.

Known [1] are lasers that have a threshold current density of less than 200 A / cm ² , and the working currents of the laser can be several milliamps. These are lasers with a narrow-gap active region several tens of angstroms thick (quantum well) inside a waveguide wide-gap layer. The width (thickness) of the waveguide layer is several tenths of a micron and is comparable with the wavelength of the laser radiation inside the crystal. So, in lasers based on GaAs-GaAlAs, where the refractive index is n≈3.2 ÷ 3.6, this wavelength can be 0.25 ÷ 0.3 microns. Several quantum wells can be located inside the waveguide layer [2], which can improve some parameters of lasers, for example, increase the radiation power and narrow the radiation pattern in a plane perpendicular to the plane of the pn junction.

Known [3] are lasers based on multipass p-n-heterostructures with a high external quantum yield, low threshold currents, and a wide range of operating currents. The indicated improvements in laser parameters are achieved by converting spontaneous emission to laser radiation. This is achieved in a pn heterostructure in which the active region of the laser has the smallest band gap. The heterostructure itself consists of wide-gap emitters of charge carriers of n- and p-types of conductivity, between which there is a narrow-gap active region, which, due to doping, has a high internal quantum yield for spontaneous emission. The contact pads in the form of strips perpendicular to the resonators (mirror) faces (chips) of the crystal occupy part of the surface of n- and p-type faces. Spontaneous radiation, repeatedly passing between the faces of a crystal as a result of its reflection from the surfaces of these faces free of contact coatings, experiences absorption in the active region with the formation of electron-hole pairs in it. These electron – hole pairs create an additional carrier concentration compared to that due to injection through the p – n junction. Born photocarriers can again participate in the process of radiative recombination. An increase in concentration due to reradiation can occur 3-5 times or more [3, 4]. This leads to a decrease in the laser threshold current by the same number of times, as well as to an increase in the laser power, an increase in the external and differential quantum yields, and an improvement in heat removal due to the dissipation of the supplied electric power in the form of useful laser radiation, which is output outside the crystal. The dynamic range in operating currents increases in comparison with the same laser, but without the phenomena of multiple transmission of radiation inside the crystal and its reradiation in the active region.

The described reemission processes can occur intensively in the active region, the thickness of which is not less than 0.1-0.5 microns (to ensure a noticeable absorption of spontaneous emission). This limits the possibility of lowering the threshold laser current density (the active region with a thickness of ~ 1 μm has a threshold current density of ~ 5-8 kA / cm ² in a non-multi-pass laser). This means that obtaining threshold current densities of less than 200 A / cm ² due to the use of multipass and re-radiation is practically impossible. The maximum thickness of the active region in structures with reradiation was limited by several diffusion lengths of minority carriers.

This technical solution by technical nature is the closest to the claimed one and is selected as a prototype.

A significant disadvantage of the prototype is the high threshold current density and, accordingly, large values of the operating current.

The technical result of the present invention is to reduce the threshold current density and decrease the operating currents.

Another technical result is an increase in the external quantum yield compared to the prototype.

These technical results were achieved in a semiconductor injection laser consisting of a doped active region in the form of a semiconductor layer with a thickness of 0.1 μm to several diffusion lengths of minority carriers located between two wide-gap layers of n- and p-types of conductivity with ohmic contacts to n- and p-layers, in which at least one layer of semiconductor material with a thickness of 20-500 angstroms (quantum well) with a band gap of 0.1 kT ÷ 300 MeV less is additionally located between wide-gap layers Than in the active region and the thickness of the active region at least two Debye lengths.

The differences of the semiconductor injection laser according to the present invention are that its structure additionally contains between wide-gap layers at least one layer of semiconductor material with a thickness of 20 ÷ 500 angstroms - a quantum well with a band gap of 0.1 kT ÷ 300 MeV less than active region, and the thickness of the active region is at least two Debye lengths.

The invention is illustrated by the drawings.

Figure 1 shows a fragment of the design of the laser according to the present invention.

FIG. 2 is a schematic diagram of a laser energy diagram illustrating a band gap distribution corresponding to layers of a laser structure (FIG. 1) according to the present invention.

The semiconductor injection laser according to the present invention includes a wide-band p-type conductivity emitter 1 made, for example, of p-GaAlAs, AlAs-30%; active region 2, made, for example, from n-GaALAs, N ~ 2 · 10 ¹⁷ cm ^-3 , AlAs-8%; a quantum well 3, located, for example, in the active region 2 and made in the form of an additional layer of semiconductor material with a thickness of 20-500 angstroms with a band gap of 0.1 kT ÷ 300 MeV less than in the active region 2; wide-gap emitter 4, made, for example, of n-GaAlAs, N ~ 5-10 ¹⁷ cm ^-3 , AlAs-30%; a substrate 5 made of n-GaAlAs, AlAs-10-40%; a layer of SiO ₂ 6, a metal coating 7, forming ohmic contacts 8 and 9 to layers 1 and 5, respectively.

The semiconductor pn heterostructure for an injection laser according to the present invention can be manufactured, for example, using molecular beam epitaxy (MBE) technology, or by a method known as metalorganic vapor phase epitaxy (MOVPE, or MOCVD-metalorganic chemical vapor deposition) [5].

In the MBE method, several streams (beams) of atoms or molecules, which are obtained by evaporation of a substance from individual heated sources, simultaneously fly in a high vacuum onto a heated substrate on which epitaxial growth of the heterostructure takes place. The whole process of heating, changes in the chemical composition of atomic beams, their deposition and growth of structure is controlled by a computer. The MBE method allows one to grow perfect single-crystal layers with a thickness from several lattice periods to several microns or more.

For the growth of heterostructures, MOS hydride technology can also be used. Here, mass transfer in a gas stream of a mixture of components of a chemical reaction with diffusion in the direction of the growth front of the heterostructure takes place. Epitaxial growth occurs on a substrate of semiconductor material heated to 600–700 ° С, near the surface of which, as a result of the pyrolysis of organometallic compounds and hydrides in the atmosphere of molecular hydrogen, the atoms necessary for growth are released. All this happens in an open type reactor at atmospheric or reduced pressure. The gas mixture is heated only near the semiconductor substrate, while the walls of the chamber remain cold, for example due to water cooling. Crystallization in the MOCVD process occurs under conditions substantially closer to thermodynamic equilibrium than under MBE. Gas flows and growth temperature are also controlled by computer. This method can grow layers with a thickness of 10 angstroms to several microns.

After fabrication of a semiconductor laser pn-heterostructure, ohmic contacts to the n- and p-surfaces of the structure are manufactured. On each surface, we applied a dielectric layer in the form of oxide SlO ₂ (maybe Si ₃ N ₄ and others) with a thickness of ~ 0.5 ÷ 1 micron. By means of photolithography, grooves were formed in the oxide layer over the entire oxide depth, parallel to one of the basic sections (for example, section <01>), with a width of 3-5 microns and a pitch of ~ 12-15 microns. On the surface formed after this operation, layers of contact metals were sprayed for the n- and p-faces. These metals were burned into a semiconductor material, and the metal-oxide interface formed a mirror. Thus, ohmic contacts were formed.

After fabricating ohmic contacts, laser crystals were obtained by puncturing (or scribing) from the structure. Gouging was carried out in the direction of the contact strips (grooves) and perpendicular to them. Chips perpendicular to the contact bands occur along the crystallographic faces of the structure. They form smooth surfaces playing the role of mirror resonator faces of the laser. The resulting crystals could be of any size. But for research, we punctured samples 300–500 microns long and 200–300 microns wide. For research, laser crystals were placed in holders with clamped metal contacts to n- and p-type surfaces.

The experimental semiconductor lasers thus obtained in accordance with the present invention operate as follows. After applying voltage in the forward direction, an electric current will flow through the laser diode. This current causes the injection of minority carriers through a p-n junction into active region 2. In active region 2, the process of recombination of charge carriers with the emission of quanta of spontaneous emission begins. A small part of this radiation (less than 1.5 ÷ 2%) will be able to go outside through the faces free of contact coatings. The rest of the radiation will be reflected inside the crystal. Radiation incident on the mirror coatings formed between the contact metal coating and the oxide on the sides of the n- and p-types of conductivity will also be reflected inside the crystal. Radiation incident on the contact strips (contact metal – semiconductor interface) can partially be absorbed here and partially also scattered inside the crystal. As a result, most of the radiation arising in the active region 2 will be reflected inside the crystal. Passing again through active region 2, this radiation will experience partial absorption in it (self-absorption) with the formation of electron-hole pairs. It should be noted that, over the lifetime of minority carriers, these electron-hole pairs recombine again with the formation of the same spontaneous emission quanta - i.e. re-emission process has occurred. If the internal quantum yield in the active region is close to 100%, then the processes of self absorption and reradiation can be repeated many times. This actually increases the concentration of carriers in active region 2 by several times. The structure has a quantum well 3 in the active region 2. Therefore, there is no need to increase the current through the laser until lasing begins in the active region (i.e., where the spontaneous emission just studied and the accumulation of charge carriers were formed). Part of the charge carriers will either drain or be “sucked” into the quantum well 3. The concentration of minority carriers in the quantum well 3 will be greater than in the active region 2 by an amount

,

,

where ΔЕ is the depth of the level in the quantum well 3, through which there is recombination of carriers in the quantum well, L is the diffusion length of the carriers, ξ is a value of the order of the Debye length (mean free path of carriers). It should be noted that the “suction” of carriers into the quantum well 3 occurs from the boundary parts of the active region 2 of length ξ. If the thickness of the active region is d> 2ξ, then in the part of the active region with a length of d-2 ξ beyond the boundaries of the 3 parts bordering the quantum well, the usual diffusion processes occur, and the effect of carrier accumulation in it is not disturbed. Since the concentration of carriers in the quantum well turns out to be A times higher than in the active region (the region of spontaneous recombination and carrier accumulation), laser generation in the quantum well should begin at a current A times lower than in the active region. However, taking into account the multiple passage of spontaneous emission inside the laser crystal and the processes of self-absorption and reradiation in the active region, the carrier concentration increases several times. This leads to the fact that the threshold current for the appearance of laser radiation in a quantum well decreases by the same factor several times. Lowering the threshold current automatically leads to an increase in the external quantum yield and an improvement in other parameters of the laser.

To reduce the operating currents, the laser can take the form, for example, of a strip mesastructure, as was shown in [3]. Mesastructures can be both on the side of n- and p-surfaces, or on both sides. Mesastructures can be made using, for example, chemical etching in the form of grooves parallel to the strip contacts and perpendicular to the resonator faces.

Literature

1. Laser a semiconducteur muni de moyens de reinjection de l'emission spontanee dans la couche active. Sermage Bernard, Brilloues. Application 2575870, Franz. declared 01/10/85, No. 8500307. Publ. 07/11/86, MKM H01S 3/18. Rzh opt. email approx. No. 7, 1987, ref. 3 107. Application 0247267, CUP. Claim 05/26/86. No. 86401106, 9, publ. 02.12.87. MKM H01S 3/19.

2. Ultrahight-Power Semiconductor Diode Laser Arrays. Peter S. Cross. Gary L. Harnagel, William Streifer, Donald R. Scifres, David F. Welch. Science, Vol. 237, II September, 1987, p. 1305-1309.

3. Bekirev W.A., Bondar S.A., Galchenkov D.V., Suris R.A., Grankin M.A., Ershova G.V., Inkin V.N., Malyshkin M.A. Laser array based on multipass p-n heterostructure. Letters in ZhTF, 1988, volume 14, issue 23. S.2140-2144.

Claims (3)

1. A semiconductor injection laser consisting of a doped active region in the form of a semiconductor layer with a thickness of 0.1 μm to several diffusion lengths of minority carriers located between two wide-gap layers of n- and p-types of conductivity, with ohmic contacts, characterized in that between At least one layer of semiconductor material with a thickness of 20 ÷ 500 angstroms (quantum well) with a band gap of 0.1 kT ÷ 300 MeV less than in the active region is grown with wide-gap layers, and the thickness active region of at least two Debye lengths. 2. The laser according to claim 1, characterized in that the active region has a width of at least 4 Debye lengths if the quantum well is completely located inside the active region. 3. The laser according to claim 1 or 2, characterized in that the side faces of the laser crystal, at least on one of the sides (either from the p-surface or from the n-surface), form mesastructures.

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