JPS6237900B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a semiconductor light emitting device that operates at a low threshold, has a stabilized transverse mode, and has good temperature characteristics.

As is well known, lasers with a so-called buried heterostructure, in which all sides of the laser active region are surrounded by a material with a wider bandgap and lower refractive index, operate at a low threshold and stabilize the transverse mode. It is one of the lasers with excellent characteristics.

FIG. 1 is a cross-sectional view of a buried heterostructure semiconductor laser, and for convenience, an InGaAsP/InP semiconductor laser will be described.

In FIG. 1, 1 is an n-InP substrate, and a groove is formed in the n-InP substrate 1 in the direction along the propagation direction of the laser beam.
An n-InGaAsP layer 2 and a p-InP layer 4 are sequentially grown thereon by liquid phase epitaxial method or the like. n-
In the growth of the InGaAsP layer 2, as shown in FIG. 1, there is no growth at the shoulder portion of the groove, and a thick crescent-shaped n-InGaAsP active region (laser active region) 3 is formed at the bottom portion of the groove. The surface of the p-InP layer 4 is coated with an insulating film 5 such as a SiO ₂ film, and the n-
A window is opened in a portion facing the InGaAsP active region 3 to form a p-side electrode 6, and an n-side electrode 7 is formed on the opposite surface of the n-InP substrate 1. In the above case, the shape of the groove cross section is smoothly curved, but in the case of a groove with a rectangular or trapezoidal cross section, meltback or n-InP buffering occurs before the growth of the n-InGaAsP layer 2. It is also possible to grow layers.

When a voltage is applied in the forward direction between the electrodes 6 and 7 of this semiconductor laser, the p-InP layer 4 changes to the n-
A carrier is injected into the InGaAsP active region 3 through the pn junction at the heterointerface. The n-InGaAsP active region 3 is composed of an n-InP substrate 1 and a p-InP layer 4 with a wider bandgap and lower refractive index.
As a result, the injected carriers and the light generated by recombination are confined within the n-InGaAsP active region 3, and as a result, this semiconductor laser oscillates at a low current value. Furthermore, if the dimensions of the laser active region are narrowed so that only the transverse zero-order mode is excited, a semiconductor laser with excellent optical characteristics without a "kink" in the light intensity-current curve can be obtained.

However, in this semiconductor laser, n-
In addition to the current flowing through the InGaAsP active region 3, there is a "leakage current" that is not involved in laser oscillation. The arrows in FIG. 1 indicate the current path of this laser diode. In addition to the current path flowing through the n-InGaAsP active region 3, the
- Current flowing to the InP substrate and from the p-InP layer 4
There is a path for "leakage current" flowing through the InGaAsP layer 2 and into the n-InP substrate 1. The former “leakage current”
flows through the pn homojunction of InP. Since the diffusion potential of this pn homojunction is higher than that of the pn heterojunction of InP-InGaAsP, the n-InGaAsP active region 3
Less than the current flowing through. Furthermore, the latter "leakage current" is still small because the current path is long.

The presence of these "leakage currents", although small, has the drawback of low efficiency and optical output saturation as the current level increases. Furthermore, the oscillation threshold has a strong temperature dependence, and oscillation does not occur at temperatures of 50° to 70° C., which is a serious problem in practical use.

This invention aims to reduce this "leakage current" as much as possible by applying reverse-biased InP pn to both sides of the active region.
By providing a joint surface, the above-mentioned drawbacks of the conventional ones are attempted to be eliminated. Hereinafter, this invention will be explained in detail using the drawings.

FIG. 2 is a sectional view showing an embodiment of the present invention. In FIG. 2, 8, 11, 12 are p-InP layers provided on the n-InP substrate 1, and n-
They are an InP layer and a p-InP layer. These layers are sequentially grown on the n-InP substrate 1 by liquid phase epitaxial method or the like. Next, a groove 13 having a depth that reaches the n-InP substrate 1 is formed by ordinary photolithography and chemical etching, and then the n-InP buffer layer 9, n-InP buffer layer 9, n-
InGaAsP layer 2 and p-InP layer 4 are grown in sequence.
At this time, in the groove 13 there is an n-InP region 10, an n-
An InGaAsP active region 3 is formed. Thereafter, an insulating film 5, a p-side electrode 6, and an n-side electrode 7 are formed in the same manner as in the conventional example shown in FIG.

Next, the operating principle of this semiconductor laser will be explained. When a voltage is applied in the forward direction between the electrodes 6 and 7 of this semiconductor laser, the p-InP layer 4 to n
- InGaAsP active region 3 is injected with carriers through the pn heterojunction and is surrounded by InP, so the carriers and light are effectively confined, and as explained in the conventional example, the optical characteristics are good and low. A threshold laser can be realized. Furthermore, regarding "leakage current" that is not involved in laser oscillation, the current path is a current flowing from the p-InP layer 4 to the n-InP region 10 via the p-InP layer 12, and a current flowing from the p-InP layer 4 to the n-InP region 10, and
Only the current flowing from layer 4 through p-InP layer 12 and n-InP layer 11 to n-InP region 10 causes n-
The current flowing through the InGaAsP layer 2 is the p-InP layer 12.
It can be ignored because the pn junction between is reverse biased. Therefore, in the path of the "leakage current", the forward biased pn junction surface is only in the extremely limited area indicated by the x mark in FIG. 2. n-InP layer 11
The "leakage current" can be further reduced by making the layer thinner and reducing the carrier concentration to increase the resistance of this layer.

Next, when the number of layers on both sides of the groove 13 is two,
The case of one layer and the above embodiment (FIG. 2) will be compared.

In Figure 3, the number of layers on both sides of the groove 13 is p-InP.
This is the case where there are two layers, layer 8 and n-InP layer 11, and the only difference from FIG. 2 is that there is no p-InP layer 12. In this case, the forward-biased pn junction surface where the "leakage current" flows spreads over a wide range as shown by the x mark.

Figure 4 shows that the number of layers on both sides of the groove 13 is p-InP.
In the case of only layer 8, the forward-biased pn junction surface where the "leakage current" flows spreads over a wide range as indicated by the x mark.

As explained above, in order to reduce "leakage current", at least three layers must be formed on both sides of the groove 13.
A pn-p junction is required.

FIG. 5 shows the relationship between threshold current I _th and temperature T for a buried heterostructure semiconductor laser, with the embodiment of the present invention shown by a solid line and the conventional example shown by a dotted line. In the case of the solid line, it can be expressed as I _th ∝exp(T/T ₀ ) (T ₀ :characteristic temperature) up to a temperature of about 100℃, whereas in the case of the dotted line, I _th suddenly increases at 30 to 40℃. increase and stop oscillation. The temperature characteristics of the embodiment of the present invention are superior to those of the conventional example.

In the above embodiment, three grooves are formed on both sides of the groove 13.
Although the embodiment is shown in which a pnp junction consisting of layers is provided, the same effects as in the above embodiments can be obtained even if there are three or more layers.

Further, in the above embodiment, an InGaAsP/InP semiconductor laser is shown, but the present invention can also be applied to a structure based on a similar principle using AlGaAs or other compound semiconductor materials.

As explained in detail above, according to the present invention, a second
It has a semiconductor crystal provided with a semiconductor layer consisting of at least three layers: a semiconductor layer of a conductivity type, a semiconductor layer of a first conductivity type thereon, and a semiconductor layer of a second conductivity type further thereon; a groove with a depth reaching the semiconductor substrate; a first semiconductor region of a first conductivity type discontinuously provided in the groove and on the semiconductor crystal; and a first semiconductor region provided on the first semiconductor region, The groove includes a second semiconductor region that becomes a laser active region, and a third semiconductor region of a second conductivity type provided above the laser active region, and the material constituting the laser active region is the same as the third semiconductor region. Since the bandgap of the semiconductor in the first semiconductor region and the third semiconductor region is narrower than that of the semiconductor in the first semiconductor region and the third semiconductor region, when the semiconductor is manufactured in two steps, the first step and the second step, the semiconductor consisting of three layers is formed in the first step. Because layers are formed, a pn junction with low leakage can be obtained, and it can operate without problems even at high temperatures of 100°C or higher, making system design easier. According to this invention, low threshold, high efficiency,
There is an advantage that a semiconductor light emitting device that oscillates in the fundamental transverse mode and has good temperature characteristics can be obtained.

[Brief explanation of the drawing]

Fig. 1 is a schematic cross-sectional view showing the structure of a conventional semiconductor laser, Fig. 2 is a schematic cross-sectional view showing an embodiment of the present invention, and Figs. 3 and 4 are comparative explanations of the operation of the embodiment of the present invention. FIG. 5 is a schematic cross-sectional view of a semiconductor laser for the purpose of the present invention, and is a diagram showing the relationship between threshold current and temperature. In the figure, 1 is n-InP substrate, 2 is n-InGaAsP
layer, 3 is n-InGaAsP active region, 4 is p-InP
layer, 5 is an insulating film, 6 and 7 are electrodes, 8 is p-InP
9 is an n-InP buffer layer, 10 is an n-InP region,
11 is an n-InP layer, 12 is a p-InP layer, and 13 is a groove.

Claims (1)

[Claims] 1 On one main surface of the first conductivity type semiconductor substrate,
It has a semiconductor crystal provided with a semiconductor layer consisting of at least three layers: a semiconductor layer of a second conductivity type, a semiconductor layer of a first conductivity type thereon, and a semiconductor layer of a second conductivity type further thereon; a groove deep enough to reach the semiconductor substrate in the crystal; a first semiconductor region of a first conductivity type discontinuously provided in the groove and on the semiconductor crystal; and a first semiconductor region provided respectively on the first semiconductor region. a second semiconductor region which serves as a laser active region in the groove, and a third semiconductor region of a second conductivity type provided on the laser active region, wherein the material constituting the laser active region is A semiconductor light emitting device characterized in that the bandgap is narrower than the semiconductor bandgap of the first semiconductor region and the third semiconductor region.